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.

remmber the adage "small hand, small pot" if it is a dry board, you should be content to check the hand down, maybe a river value bet.
in weaker games, where people are loath to dump middle pairs and such, i just value bet the whole way.
just got to know which players will call you down, and which only play good hands.

So go do some review of you HH and see what type of villain is most likely to do what in a given situation.It's the best way to learn.

Value betting thin is part of being a good player. Sometimes a scare card hits the board, but its rarely in your opponents range. For example, you are playing loose and wake up with a real hand. You raise with QQ from the CO and the fish calls from bb. the flop comes T62 rainbow. He checks, you bet. Turn 3. His range has a lot of tens in it aswell as pocket pairs. Since you are ahead of his range, you bet the turn. He calls. River is an Ace. This looks likke a scare card for QQ, and weak players would check behind. However I think you should bet the river. He only beats you if he has AT or A6 and he is calling with many tens aswell as pockets pairs and 6's. The river is a scarecard and your opponent may be expecting you to bluff this river - thats why you should value bet thin. In general when your opponent is expecting you to bluff, you should value bet thin.Grow some balls :P

If you can show me some HH i will be able to give you more precise advice.

Thank you!
I am reading about it now, it's very helpful.

Someone said something very interesting about checking instead of betting when you are in a WAWB situation;:
"Slowplaying doesn't have to be when you have a monster; the more important criterium is that the relative values of the hands involved won't change."