Here's an example of one of my wandering monster tables that I have been using:
1. Berserkers (1-8)
2. Mole Men (1-8)
3. Magic-Users (1-4; lvl 1-4)4. Giant Ferrets (1-8)
5. Bandits (1-8)
6. Crystal Statues (1-6)
7. Crab Spiders (1-4)
8. Centipedes, giant (1-8)
9. Fire Beetles (1-8)
10. Green Slime (1)
11. Adventurers (5-8)
12. Roll on “Vats lvl 2”
I'd love to elaborate about the role of each of these creatures in the dungeon, but I'll save that for another post, once more of the complex has been thoroughly explored.
But to give you an example of how all of this has worked, in the previous session, the party returned to the site of the berserkers' encampment. I decided that in ~12 rooms there would be at least one new monsters, so I rolled on the wandering monster chart, generating two magic-users. These magic-users became, after a good reaction roll, "Fred" and "George" (fake names), two companions in search a map written by Fred's great grandfather, a former student of Mazarin's.
While it was useful to have a sentence detailing each room at the beginning of the campaign, and will probably continue to do so with each dungeon level, I am now comfortable enough with the first level of the complex to run off almost pure improvisation, instead of roughly half and half preparation and making everything up on the fly.
However, this table has its limits. I've gotten to the point where I do need to rewrite the first dungeon level. The barbarians have been driven out, the Mole-Men weakened and angry, a new group of white cloaked men lead by a magic-user of at least 5th level have come in, and roughly half of the level has been explored. Instead of completely restrocking the entire dungeon level, however, I am learning that I only really need to rewrite a couple room of interest if I revamp my wandering monster chart of reflect the new inhabitants.