How much to bet on a "comeback"
I believe one of my leaks is not betting after having been calling in a hand, but ending up winner.
a) Let's say for instance I have QQ and bet preflop, an ace comes on the flop and the opponent leads out, but then he checks on turn and river.
b) Or I hit top pair and a middle kicker, and my bet on the flop is doubled up, but then again both players check the turn.
So I should bet, but in these situations I am a chicken and I check or I bet a very small amount.
How do you act in these situations?
Is there rules of thumb on how much you want to bet?
I know of course it depends on who the opponent is, but I am asking for very general advise.