The following tips were garnered from some of the more prolific area builders. We'll add more on a recurring basis.
Organization - Slash
The first step, if you are not an implementor, is to make sure the implementor likes your idea. Even if you think your idea is brilliant, and totally in line with the theme of the mud, the implementor might have other ideas. For example, I once was asked to create an Egyptian city for a MUD. The mud was medieval in theme and had the requirement that all zones would be medieval. So I spent forty hours making an elaborate medieval Egyptian city. The IMP *hated it*, and wouldn't put it in, because he wanted an ANCIENT Egyptian city with an evil pharaoh and cat-god temples and stuff. He wasn't buying an evil caliph and Islamic temples.
The next step is to draw your zone on paper. NOT with boxes and lines but a real map like you would use for a role-playing game. After you are happy with the map, THEN you can put in dots for the rooms and lines for the exits. After all of those are in, then you can number all of the rooms so you don't get confused entering the zone. You also want to consider how many rooms go into a zone. The answer is about 75-100. Some people have a hard time making zones above 30 rooms (The Troll House in Merc is 4 rooms) and some people such as myself have a hard time with smaller zones. Well, yes, your idea might be gigantic in scope and need all 300 rooms but players can't keep them all in your head and a lot of them will either avoid all zones they can't remember how to get back out of or miss some of your incredibly well written puzzles because they can't bring themselves to explore all three dozen of your eight room jungle huts.)
The next thing to consider is all of the long text descriptions of things you are going to enter. (You were going to enter nice long text descriptions of things, right? We can't have zones like the Sewers where half the rooms are This is a long, boring tunnel. This is another long, muddy, boring tunnel.). Maybe some of you are great writers and poets, but getting decent long descriptions out of me is like getting blood from a stone and I can't do more than a dozen at a sitting. So don't try. If you can't think of a decent long description just put in a placeholder like "$$$add long description here" and come back to it later. Most builder's guides will also tell you to not describe the player's emotions in your long text, as well. This is generally true, as it gets really annoying to be some 50th level arch-wizard and be told by the long description that you are afraid of some 8th level mob like Papa Smurf. If you really want to do this you might consider describing something like the room odious stench or the noxious mists that hang in the air instead. Also note that I have found that describing a place where the player feels safe and secure isn't as bad as describing how a place scares a player, such as when designing a hometown.
Plot - Lok
Slash has some good ideas on puzzles (later) for players. To his list I would like to add how you can incorporate a plot into your area. Its really pretty easy. First you visualize a story that happens in your area- lets say a princess is captured somewhere and her relatives are looking for her.
Next you drop some clues to her whereabouts by putting conversations in the 'looked at' description of mobiles. IE you can set up some mobs so that when you look at them, you get You strike up a conversation with the shopkeeper. He tells you about the rise and fall of the price of wheat. Looking at the candy jar he sighs. You ask him why, and he relates a sad tale of how he hasn't seen a little girl who used to come into his store in a long time. Granted, this isn't as elegant as mob programs, but mobprograms are not universal like looked at descriptions.
Anyway the point is that you interpret the looked at as something more than just looking, you can view it as a static conversation. You can then go around the area and throw in a few room extras leaving a trail to where the princess is. You can also put some object extras on type trash items lying about to indicate where one may find the princess. Once at the princess, you could have her relay a story of the location of a secret buried treasure in her looked at description, a nice reward for the rescuing hero. You can set up the buried treasure in a no take container with no long description with a short of 'the ground' so they get the treasure from the ground, and only know where this object is by the princesses tale. When your players stumble upon something like this, they love it. Most powermud right by it though.
Ecology and Economics - Lok
These are two things often neglected in areas. If you think about how either works in your area, then you will have a better area. For instance, an area full of carnivores with no food source makes some people wonder how do these creatures survive. By thinking of a natural food source for your mobs, and putting it in the area, you flesh out your mobs in many ways. You can even come up with plausible descriptions for outlandish mobs. Economics is important as well. Deciding what your city exports and what it imports, and how goods travel, and where banks and merchants would be located helps you make a better city. The worst cities I have seen are set up around powermudding, with every single shop selling useful items and not one shop selling anything which adds atmosphere to the area. The buffalo profoundly impacted the plains Indians life in many many ways from buffalo skin tents, to songs of the hunt.
Keywords and C - Slash
Please give NPCs keywords for all of the names a player might call them. If you have a shopkeeper named Thorik the Dwarf you had better give him keywords of shopkeeper, thorik, and dwarf. It really sucks when you are in a room and it says Thorik the Dwarf is here and you say kill thorik and nothing happens, then you say kill dwarf and nothing happens, and the mud is only accepting kill shopkeeper. Understanding keywords is really fun, as there are a lot of things that can be done with them. In the High Tower of Sorcery there is an illusionist. He is in a room called The Illusionist's room. His short description says A large dragon. But if you type kill dragon nothing happens. You have to do a kill illusionist to get him. See, he isn't a dragon, so kill dragon doesn't work. Yes, this is silly. Be careful with your keywords.
There isn't a lot of things you can do with mobiles, unless you either have the MOBprograms on your MUD or can code spec-functions in C. There is the old gag of not giving the mobile reasonable keywords, but that gets old quick. My advice is to learn to write spec-funs or MOBprograms.
Another trick if you happen to be writing C code is to write commands that only work if the mobile is in the room. Most people are familiar with banker mobiles, but you can do other stuff as well. I made a crazy old man that can identify equipment for money by having an identify command that only worked if he was in the room.
Shopkeeping has some tricks to it as well. You can make the shopkeeper wander, so players have to track him/it down to get the object they need for the quest. This can be a key he sells, where the key is disguised as something else like a ticket.
You can have two mobiles that look similar but have different items or one is a shopkeeper and the other isn't. That is a lot of fun.
Longs and shorts - Lok
Check out the ROM docs- they say consider dropping longs on mobiles which are sentinel and putting the description in the long of the room. This could be cool for shopkeepers (I think that's how they did Cordreas Heart), and it would be a great trick to make 'truly hidden' mobs. If you dropped the short description too, then the mob shouldn't show up on scan- a really neat trick indeed.
Drop the long description - Lok
Try omitting the long description on a stationary, no take object, and put it instead in the room description. This encourages players to read the room description to find all the containers in the room. (A good example of this hint is the 'rathcor' in Chapel Catacombs in stock Merc. - Slash)
Weight - Slash
Sometimes you can make the object have a takable bit, but give it a huge weight instead, like 30000kg. This causes the server to say It is too heavy for you instead of 'You can't take that'. Good for statues, and funny when you see players dropping all their stuff and stripping naked to get the things. Also fun for the immortals because they can carry the stuff and move it around for fun.
Useless objects - Slash
I'm sure you already have figured out about all the +++ magic swords and stuff for your zone, so I'm not going to talk about those. I'm going to talk about useless objects. These are a lot of fun. You put them all over the place to distract players. A good example of this is in Thalos, where there are these +stat rocks on the ground. Some of the rocks do good things when held, but others do nothing or have a - effect. Don't totally go to town on this, outfitting every mob with 18 different reset positions of normal objects, but try to have some fun. Lets say you have a witch-doctor with a +3 int magic tribal mask. Give him a some jungle magic as well, maybe a necklace made of human teeth (non-magical), a non-magical juju bag with maybe a chicken's foot in it that is a staff of create water, an herbal potion of refresh, maybe dress him in a zebra-skin loincloth (non-magical). One important thing to consider when doing this is some servers like Merc will auto-adjust the value of weapons and armor. This means that if you have a high-level queen wearing a frilly lace dress it will have the same armor class as platemail if you make it of object type armor. So what you do is make the stuff of object-type junk but give it a wear bit.
Containers - Slash
Containers are great fun, especially when they don't look like containers. For example, you can make a bunch of trees for your zone and only one of them gets a magic apple reset into it. You can also make a coat with pockets by giving it a wield position.
Pills - Slash
Pills are also fun. A pill is just a potion that is eaten instead of being quaffed. I personally like to equip non-humans with pills instead of potions. Pills don't have to be pills, but can be anything a player might conceivably stick into the mouth, such as eye of newt or fried bat-wing. You can put both beneficial spells or harmful spells, or both, into a pill. One nice thing about pills is that you can't eat one when you are totally full of food, so that those lazy adventurers who keep themselves constantly stuffed with food can't use them.
Keys - Slash
Keys can be disguised to look like something other than keys. For example, you could make a king's crown that can actually be worn, but is also a key.
Wear locations - Slash
A good technique is to create non-armor equipment with wield positions to surprise the player, for example a ring of water walking which is a boat with a ring wield.
NPC spells - Slash
If you can code spells you can make some really interesting objects by having NPC only spells, such as the guns in Mega City 1.
Navigating by exit descriptions - Lok
In the seas area I wrote, there are many many identical rooms, some of which border on 'shoals' rooms, rooms with no exits for which you must recall. Anyway the players can get through the area easily in the daytime, but at night 'exits' doesn't work. However I put in exit descriptions in each direction so that a player could look in a direction and see if there was land, seas, shallows, or shoals in that direction. This is how they sailed. Dumb players would wait til morning. Smart players would navigate by looking at compass points. The point is this- those exit descriptions are always there, and you can put clues and paths and other hints in them to help the player. For instance the great pyramid uses exit descriptions to tell the player whether a path in any given direction is safe.
2 exit moron mazes - Lok
I have noticed a tendency in many muds now to abandon the standard merc map and remap their world. Often this is done poorly by the use of 2 exit moron mazes. By this I mean a series of 2 exit rooms which define a path which always leads somewhere. I have seen this in alot of new areas as well. A writer will make a huge segment of his/her area as a 2 exit moron maze, so the player is forced along a path throughout the area. These moron mazes really don't provide much of a mapping challenge, or variety to the area. Its very easy to beef up your room count with a 10 room road. It is much more challenging to make most of your rooms 3 and 4 exit rooms, where some thought is involved and there is a chance of a person getting lost. I think its much better to remap and connect through areas, not paths.
Randoms - Slash
Ok, the basic technique you see in most Mercs is random exits. What you do is label a room so that its exits are randomly switched. This makes it impossible to follow a set course through. In stock Mercs you see this best in The Great Eastern Desert. There are nine rooms labeled The Great Eastern Desert in this zone. All of the exits in the nine rooms go to other The Great Eastern Desert rooms, except one that takes you back to Midgaard, one to a dragon, one to a nomad camp, and one to MegaCity1. Now, if you have never played a MUD with random exits you are probably thinking that nine rooms isn't very many and someone can easily get to any of the four exits with a little poking around. WRONG! Nine random rooms is a LOT of rooms. I have burned hundreds of moves just trying to get to the MegaCity. It is REALLY hard, especially at night when you can't use the exits command to see if you are bordering the room you are looking for. So be very sparing when you use this technique.
One-way exits - Slash
Another popular technique is the one way exit. With one way exits, adventurers cannot backtrack and so must press forward in your zone. An example of this is the Galaxy zone. You enter through a crystal ball in one zone and later leave through a rope from the clouds. Another technique is solitary and private rooms. A private room only fits one character, and solitary only fits one. Note that we are talking characters, not players. So a solitary room can be used to force players to fight mobiles in single combat.
Secret doors - Slash
Secret doors are a popular technique. Note that in Merc a door must be in one of the six cardinal directions (N, S, E, W, U, D), so that players who are on the lookout for secret doors just check all directions that don't have normal exits. All doors in Merc can have special names, so you could make a room with a life-like tapestry on the wall and the player could open tapestry; south to go into the tapestry.
Mob confinement - Lok
Many areas have no mob confinement, and all mobs wander all over the place, or even worse, are frozen in sentinel stances. If your mud has a stay terrain bit, you can use terrain to define a 'range' for your mob. If you do not have that bit, consider using no mobs room flags to limit the range. You should always mob lock your area by making the connection point to your area a no mob space. Good places for no mob spaces are stairways between levels, gates going into and out of cities, and trail junctions/or intersections. If you do this, you can slice up your area into several smaller ranges into which you load mobs and let them wander.
Mazes and paths - Lok
I played in a great area called the Jungle by Landru. He had it set up so that there was a path through it. The path, however was not obvious. You had to find it by looking at extras in the room descriptions. There were hints as to which way the path lead, like blaze marks on trees, or sticks pointing in certain directions. Often you would have to go 3 extras deep to find the clue. Anyway in any given room there were about 4 or more exits- 2 were on the path, and 2 lead into a nasty random maze of a jungle. Smart players could follow the path to a jungle city, and stupid ones would wander aimlessly forever in the jungle.
Avoiding Mob pileup - Lok
Mob pileup happens when you have 2 or more resets of the same mob in an area. Players kill mobs at a reset further down the list, and the game resets the mob at a reset at the top of the list where there are already mobs. So you get all your mobs loaded in one spot, and the area rapidly becomes unplayable. (ROM avoids this problem with a local limit field as a 5th number on a M reset, where it checks to see if the local limit is already met prior to loading another mob). The cure for this is simple. Copy the mob to a new vnum, one for each reset. Midgaard for instance, has many many versions of cityguards, so that each guard loads at its proper gate.