Quoted By:
On occasion, add high levels of detail to random unimportant objects. Two extra sentences on a statue that nobody needs to give a damn about doesn't take much time, but it's enough time to catch the "the DM is paying attention to it, it must be trapped or important" one.
Switch monster stats on a regular basis. If the players have the HP for kobolds memorized, you can still use kobolds if you call them beemen.
Keep the players together. Dealing with out of character knowledge is a bitch, and so is handling multiple parties in the first place. If they must split up, make sure circumstances change wildly between visits.
Either don't use so damn many tropes, or use genre-aware villains.