The thumb rule goes: play big pots with big hands, keep the pot small with marginal hands.
If you have a marginal hand, like a pair, but you think you're ahead based on how the betting has gone, this is a marginal situation. If you're right and you win a small pot with your pair, that's great.
But if you're wrong and you lose a big pot with nothing but a pair, that's reckless. A pair isn't worth a big risk.
On the other hand, if you have a full house, that's a big hand. You want to play a really big pot, preferably for your whole stack if you can. Once in a while you'll be up against a better hand and lose, but that loss will be covered by all the times you win against a smaller full, a flush or - why not - a recklessly played pair.
Keeping the pot small
So how do you control the pot size? If you want to keep the pot down, the primary trick is to skip a betting round.
In no limit and pot limit poker, bets tend to increase "exponentially" on each round of betting. Before the flop you bet maybe 3BB. On the flop it'll be more like 6BB, on the turn 20BB and on the river maybe 60BB.
That's a total of 89BB. Now, if you can achieve that anyone of those rounds is checked down, it's always the last bet in the series that will fall out, that is, the 60BB bet.
So, instead of losing 89BB with your puny pair, you now lose 29BB. Much better for your bankroll.
Achieving this is easier if you're in late position. If you raised before the flop, players that act before you on the flop will often check to you. Then you can check behind and you have actively controlled the pot size. Now you can call down two bets on the turn and river. Or if the opposition checks again, you can do the betting.
(Of course, when you check behind you give opponents a free card and they may hit a dream card. This is a risk that you need to take into consideration.)
If you're in early position, you can check and hope that the opposition checks behind. If they bet, you'll have to take it from there. Check-calling is still much better for the pot size than betting out and facing a re-raise.
Building a big pot
If, on the other hand, you want to build a big pot for your big hand, you must avoid that a betting round is checked down.
If you're in early position, this means that you should avoid slow playing your big hand. If you check and it's checked behind you, chances are you'll never be able to make that 60BB bet on the river.
(Of course, in no limit you can always make the bet, but if you bet three times the pot, worse hands won't pay you off.)
So, in early position you'd bet out almost every time. (An occasional check is motivated by the need to mix up your game.)
If you're in late position, the optimal action depends on your read of the situation. If it's checked to you, you'll bet almost automatically, just as above.
If an early position bets into you, you must consider how you'll get the most total chips into the pot. Will she call a raise? Then go ahead and raise.
But if you're afraid that she'll fold to a raise, a flat call may cause her to bet again on the next street, thinking that you're weak.
Context sensitive plays
There are a range of behaviors that may cause your opponents to bet or not to bet, depending on your desires and the betting context.
For instance, if you're in early position with a marginal hand and you feel that a check is very likely to induce a sizeable bet from your opponent, you can try to make an undersized bet. Sometimes a small bet (a control bet) can plant some doubt in the opponent and he may choose to just call.
In other cases, an undersized bet can be the exact right medicine to induce a huge bet from the opposition. Your tiny bet might look like a weakness, a control bet. Your opponent rams the pot to scare you off, and you win big with your big hand.
To predict this kind of reactions, you have to be in the game. You have to feel the momentum of the game against your face. Of course, you won't always succeed in your intents. Opponents have wills of their own, and they will be playing their hands in the best way they can, just like you.
It's a struggle, and it's called poker, and you love it.