Online:
1) Patience early on in a tournament.
2) Don't try to
bluff a bad player because they don't know when they should be beat.
Live:
1) On the flop, watch the other players as the cards are revealed. You can look at the cards later, but you only have one chance to see their reaction to the flop. If they stare at the flop that means it usually didn't help them. If they quickly glance at their chips, they like the flop and plan on betting.
2) Pick your targets. Pick on/exploit the weak players and avoid tangling with the strong ones.