I love, love, love Caverns of Thracia. It was the tent-pole dungeon for my
White Sandbox campaign, and was also a big part of the Borderlands campaign that was the other source for Adventurer Conqueror King. Reaching the climax took 22 sessions, 37 players, and 55 characters (
link contains spoilers).
Here is my advice:
1. If you're working with the Judges' Guild original, write in on the maps (or a photocopy thereof) where each staircase and off-map connection goes. There are lots of these inter-level connections - part of the greatness of the module - and it's
not always clear. The Necromancer version cleans this up a little.
2. Again if you're using the JG original and OD&D or 1E type rules, I found that there weren't enough treasures in the upper levels to let characters advance (GP = XP, remember) to the point where they could tackle the lower levels. What I'd recommend is using the rumor in the rumor table where a collector offers 2d100 for Thracian artifacts. Seeding the dungeon liberally with such artifacts will help get the ages of the dungeon across to the players, and create connections between town and dungeon. Speaking of which...
3. I created a town to be within a day's travel of the dungeons, and was super glad I did. 22 sessions of dungeon adventure would have been a lot less interesting without things that happened there having repercussions in town (I extended the factions to have overlap in town, so that worshippers of the Dark One became upset when their temple artifacts were sold to the town's merchants, etc.)
Also worth checking out are Justin Alexander's
essays about Caverns of Thracia; spoilers here as well.