Check Raising the Flop

[Sciolist]

I am wondering when it's sensible to check/raise the flop. Whenever I do it, it's generally because someone is too aggressive/c-bets too often. It almost always signifies that I have an OK hand or a bluff. If I have a STRONG hand, I'm calling and letting him bet again. If I have a set or whatever, I'm probably leading out as a check/raise loses value from hands I'd get it from by either check/calling or betting.

Where do you lot check/raise the flop?