Have you done Web searches on "cave map" and "cave photo"? Have you checked WOTC's Maps-a-Week and Dungeon Magazine's back issues? Use whatever scale you like. Don't forget to pillage your other adventures for maps (and alter the scale if you need to): got Erelhei Cinlu?
http://www.wizards.com/dnd/DnD_Features_Nav.asp?L=MW
http://www.wizards.com/default.asp?x=dungeon/backissues
Don't forget the public library; checking out a few books on real-world caves, speleology, and spelunking should prove very worthwhile. Got time to visit/tour a nearby cave? Only you know how the PCs will respond to the environment, so as you make ad hoc rulings in response it will be useful to know what your particular caves are like in as much detail as you think you'll need. Real-world caves vary dramatically in just about every meaningful way, so you might find it easiest to pick a basic cave type and stick with it.
Of course, the beautiful thing about natural formations, map-wise, is that you can get away with basically scribbling a map and just going with it. After all, the players aren't going to see the map you draw and aren't necessarily interested in whether you've got all the nooks and crannies right. But having a good idea of how your cave was formed and what general shape that sort of cave tends to take will make everything more believable.
Be very aware of what the players can see and hear. How far can the characters see? What's in their light radius? Low-light vision radius? Darkvision radius? Does anything glow? What sorts of geologic features are present within view? Is the air fresh or stale? Does the surrounding earth reflect and magnify sound or absorb and dampen it? (Impose a penalty or bonus on Move Silently checks.)
On the flip side, how much noise, light, and odor do the characters emit? Many cave residents will have the equivalent of either tremorsense, blindsight, or darkvision to compensate for darkness or even total blindness. Bats, though not themselves blind, possess blindsight based on sounds the bats themselves make; other creatures might "home in" on vibrations, sounds, or scents they encounter. If the characters are wandering through untouched, pristine caverns, they should expect a different welcome than if they are wandering through often-used (even patrolled!) passages.
How do the characters plan on keeping track of time? Day and night are more or less indistinguishable underground, to say nothing of morning and afternoon. If they have no plan, be ready, the first time the players need to know what the time is or how much time has passed, to tell them their characters haven't the faintest idea. Or have someone make a Wis check and use the result to gauge how accurate the characters' sense of time is. (The DC ought to go up for a while as the PCs stay below ground, then go back down after they're used to it.) This will affect spellcasters mostly.
Do the caves have any non-monstrous residents? How do these creatures live? The underground ecology is quite different from the aboveground, and especially so in a fantasy world -- but some organism (fungus, plant, animal?) almost certainly fills the role of "food crop" and another (animal, beast, humanoid?) fills the role of "top predator." Where are these organisms, how common are they, and how do they (and other organisms) interact? Including the wildlife in your descriptions tends to boost verisimilitude. How plentiful is water? Can the characters find water while traveling or is the water supply a real worry?
You'll want to decide whether the cave is easy to navigate (full movement), a little tricky (2/3 movement), or really tortuous (1/3 or less movement). Do the characters have enough room to move around freely? If not, you'll need to penalize certain skill checks and attack rolls. If you've got characters with nice high Climb bonuses, put them to the test. Is the terrain treacherous enough the characters will want to rope themselves together? Devise a reasonable series of Dex and/or Str checks for when one of them falls off the narrow ledge.
Other factors are dependent on what's going on between the sentient underground races. Duergar, svirfneblin, illithids, aboleths, drow, pech, derro, magmins, kuo-toa, humans, and more may all be vying for space, food, slaves, luminous fungi, ore, gems, trading rights, or some other limited resource. Who are the dominant folk in a given area, and why? How do they treat surface-dwellers? Do they even know anything about the surface world, or is it all just one long crazy myth to them?
HTH. HAND. Good luck!
