Man, I hope other people do this. I dunno if you do, if I just missed the memo and rediscovered this great fucking tip myself, or what, but if not, if this is not something that everyone does, listen up:
When your PCs are in a dungeon, get yourself a starkly different colored die. Match it to the period of random encounters (a d6 will do just fine for every hour) and turn it to represent ten-minute segments. GOD, IT'S GOOD.