I know this sounds kinda obvious, but you should be playing tight enough so that the hands you do play, given your position, can win (and win a big pot). Your goal is to win money, not pots.

The goal is ALSO to keep your stack. You want to keep yourself out of positions where you're bleeding chips off (chasing bad draws, weak cards (like QJ) out of position, etc), so you want to play tight.

On the other hand, your big hands you want to play aggressively, betting and raising. (The real key here is that so many people play crappy hands, when you only play good hands you're going to show a long-term profit.)

You can't be afraid, either. Don't be afraid to lose. And don't be afraid to win.