thanks Steve I was trying to give myself a general rule and not looking at all of the other factors
I agree with Steve that there are many factors to the decision. Tee-tee, you will find that almost every question in poker can be answered with the phrase "It depends."
However, I'll give you a "general rule" I use myself for most post-flop situations:
"When you have a dangerous board, make 'em pay dearly for the next card."
Dangerous board = 2 cards of a suit or 2 connected cards for a straight. Both of those would make a REALLY dangerous board.
Pay dearly = about a pot-sized bet would do. You give one player 2-1 odds to continue. (IE: If you bet $2 into a $2 pot, the pot is $4 and it's $2 for them to continue. 4-2 = 2-1. ) Now if you have more than one player in the hand with you, you may want to bet more than the pot. (I won't get into the numbers showing why, so as not to confuse you just yet. )
Anyway, I hope that helps. Also, you oughta read (or at least skim) the Lee Jones book that Freak got from me. It's a Limit Hold'em book and I haven't read it yet, but I bet there is a chapter in there that covers "bet sizing". Cheers.