There are alot of ways to keep dungeon crawling from becoming stale and the three most important are setting, setting, and setting.
Any dungeon, no matter how varied, is going to become boring to an experienced player unless he is given more than just monsters to fight and traps to find.
Before drawing up a dungeon, you need to ask alot of questions. Who built the dungeon? When? For what purpose? Is it still serving that purpose? How have latter occupants altered the dungeon? What features of its older use does it retain? What is its current purpose in the society? What secret does it hide that will help advance the larger story you want to tell? Have adventurers been here before? If so, how have they altered the dungeon? If it is an eyesore or source of trouble, why haven't figures more powerful than the PC's leveled the place a long time ago? If it is rumored to contain treasure, why hasn't that treasure been looted by figures more powerful than the PC's a long time ago? Asking these questions repeatedly will give you fresh ideas.
Each room in the dungeon should address and help answer the questions raised in the overall vision of the dungeon. Rooms should give clues as to older functions (if any) as well as show signs of how they were used. Always dress every room. Rooms have odors, age, cleanliness, trash, furnishings, decorations, floors, ceilings, often inhabitents (if only normal rats, spiders, earthworms, snails, millipeedes, ants, cockroachs, lice, flies, etc.), light (or its absence), sometimes palpable aura's, sometimes weather (temperature, drafts, humidity, haze), construction materials, and so forth. Mosiacs, frescos, and carvings should reveal history when appropriate. Consult or construct lists when you are stuck for ideas. There should be _AT LEAST_ one unobvious and hidden feature (of some size even if largely unimportant) in every room, and preferably at least _ONE OR TWO_ things that require some skill check (to find things, reveal information, negotiate an obstacle, etc.). There should also be at least 1-4 dressings with no real significance except to provide atmosphere.
Once you get the hang of it, you can elaborate on features in the middle of play, but better to over write a room than under write it and be stuttering to try to come up with something on the spur of the moment. Nothing will tip off players that the room is unimportant (ei doesn't contain treasure or information) than hasty descriptions and a paucity of interesting features.
Keep truly empty rooms to a minimum. How many rooms are originally designed to be featureless, still used for that purpose and regularly cleaned to keep features from accumulating? Reserve featureless rooms for disentigrate traps, areas swept by gelatinous cubes, etc. Otherwise, why waste your time putting an uninteresting place on the map?
Work steadily to build a desired atmosphere. Keep your players imagining thier surroundings. Encourage your players to interact with the surroundings.
Keep dungeons small. Six to fifteen rooms is ideal for most dungeons. More important areas should have as many as 25. In no circumstance should a dungeon have more than about 70 areas. If it does or should (many real buildings have 100's of rooms), break it down into a series of connected dungeons and cordon off areas with obvious divisions. These need not (and probably should rarely) be staircases to lower levels. Don't be afraid to make dungeons with 4 or fewer rooms where brief expeditions can be made. Often spending time making rooms interesting is better than spending time making more rooms.
You should never design a dungeon without at least one room which is sufficiently architecturally complex as to be hard to describe without showing the players a map. Otherwise you are going to be stuck in 20'x30' rectangular room syndrome. The average apartment is not so simple. (Think about the 'L' or 'T' shaped rooms, double rectangles, split level rooms, rooms with direct access to corridors, rooms with partial walls between them, landings with staircases leading off and onto them, balconies, etc.)
Always keep in mind that any sequence of areas which can be bound in either space or time (or both) can be used as a dungeon. One room (the common room of a tavern, the basement of a haunted house, a hut under seige, can suffice for a dugeon if over the course of a limited time a series of encounters take place there. Dungeons do not have to be isolated subterrainian ruined dwelling places of evil. They can be palaces in which only some of the inhabitents are nefarious, churchs with haunted crypts, tenement buildings which are on fire, opera houses, theaters, and libraries after dark, taverns, castles under seige, mansions in which the characters are guests, hollow trees, gardens, asylums, bath houses having plumbing problems, sewers, graveyards, lairs carved out by giant moles, copses and hollows surrounding evil shrines in public places, boats, and so on and so forth.
When using ruins, don't focus solely on the standard dwarven mine, old castle, wizards tower, ancient temple of evil, and trap filled tomb. Idealy, those old standby's ought to be present AS INTERESTING CHANGES FROM THE USUAL, and reserved for your more important challenges when you can make the most of what the setting provides. Give players a diet of aqueducts, arenas, mansions, mills, lighthouses, hospitals, monestaries, covered bridges, barns, hermit's retreats, farm houses, dry docks, quarry's, wells, monuments, gnomish clockworks, hobbit holes, etc. etc. etc.
Frequently reference books on architecture to get inspiration for floor plans, new types of buildings, etc. No matter how imaginative you are, you are going to have a hard time mapping out better buildings than those made by full time architects with brillant imaginations of their own. History is filled with great buildings.
Traps tend to be overused and poorly used dungeon features. Don't get me wrong, I love traps and consider them essential to any large well rounded dungeon, but place your traps only with great thought. Someone built those traps, paid for those traps, and _LIVES_ with the effects of having all those traps about. Traps should only be placed in well traveled areas in very limited numbers (probably no more than 1 per dungeon) - and in general these traps should be of the non-lethal variety (snares, alarms, nets, small pits, lassos, bear traps and similar, shutes, etc.) Place the other traps only in places that are infrequently visited, preferably in places that no one is supposed to visit (false leads, corridors that go no where, doors that don't open), or which can be bypassed by those in the know (broad corridors flanked by untrapped (but guarded) side corridors), short cuts not meant to be used, etc. There must be some logic to the traps or your players will get the feeling that they are just toys being arbitarily used for your amusement.
When present, make sure your trap inspires respect appropriate for the level of your player's.
Finally, make sure that the players know why they are going (or going back) to the dungeon. Try as hard as you can to make it some reason other than the desire to loot it. Strive to reach a point were play is story driven not dungeon driven. Many of the best dungeons are not introduced as dungeons. Instead, the players begin by treating them as a non-dungeon environment, only to discover some (or many) sessions latter, that there is some reason to explore them more closely and not all of the inhabitants where as friendly as they appeared. In this fashion, the dungeon begins to have a character that the players know and can interact with.
NEVER DESCRIBE THINGS IN GAME TERMS UNLESS YOU MUST FOR THE SAKE OF PERFECT CLARITY.
Work to make your descriptions lively. Write down a few sentences of description ahead of time and read it (or work up from it) if you must.
