Since Shieldhaven brought it up in the comments on this post, I should talk a little about what I do when it comes to dungeon-running.
I grew up playing my dad's AD&D games, which were always dungeons. I think I've only run one game that was not a dungeon, and that one was a one shot for a single player. Contrariwise, except for my dad's game, I don't think I've played another D&D game that has been primarily dungeon based. So I guess it is still a little odd that for me, when I am running a game, dungeon exploration is pretty much everything. Which isn't to say that the world above doesn't matter-- it certainly does. Just that the players won't typically be interacting with it directly; at least, not for some time.
The part that can be chalked up to laziness is where I like the constraints that dungeons put on the party's immediate actions, at game start. There's this map, see? There's only so many directions they can go. I don't suddenly have to gen up a bunch of stuff I'm not prepared for (unless I haven't read my notes in a while, which has certainly happened), or come up with a more artificial way to use something I'd planned. What's there is there.
That said, no one really wants to go through another boring hall after boring hall, fighting incongruous monsters and improbable traps that don't seem to have any reason to be there. Also, there's the problem where you find a bunch of loot that is difficult to impossible to spend, because there's not a ton of good shopping in the underground. Usually. So here's a list of things I think about when I'm preparing a dungeoncrawl campaign.
Who Lived There Before, And Who Lives There Now?
The first two things to consider are who the structure was built for previously and to what purpose (which the DMG will help you with), and what's taken their place. This is really two items, but the first one is pretty straightforward: the contents of the dungeon should make sense for the dungeon's purpose. If it was a temple, there should be religious iconography. If it was a literal prison-dungeon, there should be some remnants of the horrible tortures and privations that took place here. My personal favorite, however, is the (probably) Abandoned Wizard Lair(tm). This setup justifies all kinds of crazy shit, including whatever you want to stock the place with now. Here, you mostly want to say something about who the Wizard was-- or still is-- with the contents of the dungeon. Want to justify a full range of asshole jellies and lurkers above? Maybe your wizard was really into turning people he didn't like into hostile sludge. Mimics and floating swords indicate someone who did a lot of item enchantment... maybe they created more and more intelligent items until one of them managed to win an ego contest.
Beyond the mechanics, the overall decor of the place should speak to the kind of person we're talking about here... decadent? Ascetic? Workmanlike and studious? Did they have apprentices? Merchants? Lavish Parties? Fey adherents? Summoned demonic servants? There's a lot you can imply with judicious window dressing.
As I suggested above, the previous inhabitants do a lot to imply who or what is likely to have moved in in their absence. For example, a bunch of dumb kobolds aren't super likely to have moved into the Traps-Mage Haran Roeh's Super Deadly And Stunning Whirling-Blades-Of-Death-Maze-- at least, not if they came in by the front door, unless there's a huge pile of them lying dead at the entrance, and the traps themselves are smashed to bits. Sentient beings that _aren't_ a leftover experiment should be found in relatively insecure locations, unless they've placed or repaired the protections themselves. That said...
Have Potentially Friendly Sentient Beings (who aren't the party) Present in the Dungeon.I have a lot of love for running into NPC adventuring parties, personally, but sometimes you want the place to have been undiscovered before the PCs get there. Well, "undiscovered". I think primarily uninhabited dungeons are great for mini-adventures in the course of a campaign, but if the bulk of your game is going to be the dungeon, there really needs to be something vaguely recognizable as people there. This can be a tribe of orcs or goblins, or better yet, a tribe of orcs skirmishing for territory with a tribe of goblins, that the PCs need to resolve to continue. This can be deep-dwelling nomads of whatever kind, were creatures, transmogrified humans, sentient jellies, modron-- whatever, so long as the PCs can come and have a chat with them in-between the violence. Or maybe they just charge in and murder them all, if that makes them happy. But there should be a sense of disruption if they do that; and an indication that they kinda just burst into these folks's living room.
Why would you want to do this? Apart from breaking up the violence, this gives you a source for information about the dungeon (for a price), gear upgrades/improvements (that pesky, "where will I spend my gold if I'm underground?" problem), puzzles, and other suchlike. It also gives the dungeon a sense of being a living, breathing place that exists in the world. I mean, if you want to run something specifically about the idea of being perfectly frozen in time, that's cool, and I'm now thinking about how I'd make a metastory like that work, but once again, I think it'd be a good side-dish to the main campaign, rather than a campaign in and of itself.
Have some baller loot, but also something to do with it.As I just mentioned, if you're handing out coin, yet your campaign is going deeper and deeper underground (or up in the air, the soaring tower dungeon is a pretty awesome idea too), your players might be a little confused over time with what to do with all that filthy lucre they're gathering. If you're like me, then you have at least one wise acre in your party who will interject periodically with the exact value in weight of a given number of gold pieces, possibly narrowly avoiding blunt force trauma with a printout of message board debates on encumbrance rules.
Obviously, handing out usable loot is a good idea- magic items, better armor, spells, and training treasure (a whole other post I wist) are immediately useful and happy making. But what about straight-up coinage and non-magical items of value? Let's take a look at that:
1) provide an easy way in and out of the dungeon. - this has its good points and its bad points. The less ground the party needs to cover to get in and out, the more likely they are to come back after they've gone to the surface and dumped a bunch of cash. However, you have to be careful with this, as depending on the method they're using, theoretically, other NPCs on the surface might start to get curious as to where these people are going and coming from that they have all this delightful moolah. On the other hand, that could be a pretty good story hook. Just remember that any method of egress left unguarded is likely to lead to cleared areas becoming most decidedly unclear. However, magical teleportation/hearthstone-like mechanics are a way around this, for sure.
2) Aforementioned NPCs - having NPCs in the dungeon who will trade the party's cash for goods, info, or other suchlike is always a winner to me. I talked a little about that above, but for verisimilitude (avoiding the JRPG trope of WTF is this merchant doing way down here in this dangerous dungeon!?), unless you're mad gonzo on purpose, at least give a semi-plausible excuse-- like the decimated party just trying to escape as quickly as possible, or monsters who use surface coins and valuables as tradable goods in their subterranean economy, or...
3) The coin-operated puzzle - Finally, you can come up with puzzles that consume cash as a part of their solution: metal eating monsters who can be distracted with coins, to avoid them trying to munch on the party's plate armor, traps that can be disarmed by inserting metal disks of a certain size in strategic locales, or, as Shieldhaven did once _not_ in a dungeon, a ritual that requires slagging a large amount of gold or electrum to accomplish a goal. This one can be fun and interesting, but 1) its hard to make it siphon off a large amount of cash without it feeling punitive and 2) the reward should be commiserate. After all, what is money? The ability to measure the relative value of rewards, and most importantly, the ability to choose your own reward in exchange for such currency. This method potentially sacrifices some of the choice, which has its own value, so if that is the case, be sure to make up for it.
4) The Skee-ball option- As a follow up to the latter, consider that coins are merely one potential form of currency, and one that potentially has less value when you're deep in some wizard's abandoned lair with the back door to the fiery pits of doom (based on a true story). So if you can't spend _coins_ with the denziens of the dungeon, maybe they value something else, and maybe there's either a device, or some of the above options where you can trade hard currency for, well- carnival tickets to get the prize you actually want. Man, this illustrates the actual bizarreness of the carnival midway as game mechanic system, huh? Trade cashy money for tokens, then spend the tokens on $activity, then get tickets, then trade for random stuffed animals you suddenly REALLY want. So how to couch this in a game example?
One idea: the players come upon a laboratory in the Wizard Whackjob's Nefarious Multi-Dimensional Demsene, which still seems to be in working order. There are a series of vials there containing various alchemical solutions, and the players learn, through skill checks/trial and error, that feeding coins of various metals to the solutions will generate a slew of mystical materials that, incidentally, the tribe of deep-dwelling but friendly dwarven bone-crafters especially covet. Inquiring with said tribe informs the party that they will happily make Super Awesome Carved Bone Gear and maybe even share some spells with your spellbook having types, in exchange for materials which hey! They can now suddenly craft (why don't the dwarves use the lab themselves? Maybe it's in a different part of the dungeon with some horrid beast the party has to deal with in between- or they just don't know about it. Come up with something- it's not complicated). Anyway, this preserves some reward choice, while also giving players something potentially skill based to do with the money. Bonus points if there's multiple groups of monsters/folks who are after the token materials, and whom the PCs have to keep in balance. Fun times!