I am aware of the idea of a continuation bet, where you bet again on the flop, after a pre-flop raise. However, I'm not that sure on when I should and shouldn't do it. I tend to do it if I get the idea (from other players checking, say), that other players have missed the flop, but it would be useful to have more specific things to look for.
Also, how big should continuation bets be? I've heard between half and the full pot as a guide, with the higher end for if the flop has high draw potential (lots of hearts, say). Is that reasonable?
How much does it make a difference if I have hit the flop or not? Also, how much of a difference is there between cash games and tournaments?