[Charlemagne]

I usually raise about potsize PF. The amount of $$ allready in the pot before you do anything plus twice the amount you have to put in just to call.
But besides that, if i miss i usually bet a little under the pot on the flop. 2/3 should be about right. But i dont think you should fire a second bullet unless you think he was on a draw which didnt turn. Someone with a small pair who is stupid enough to call the first bullet will prabably call the second as well. So a second bullet should only be after much consideration, in my opinion.
Note that the first bullet you fire on the flop isn't nessecarily a steal. Just because you missed doesn't mean he didn't.