I heard this tip while watching Poker on tv the other night and since putting it into practice I have done better and been making nice bank at every kind of Hold'Em game: cash games, heads-up SnG, single-table SnG, tournys. I'm playing at Full-Tilt btw if anyone wants to play with me
Pre-flop, NEVER call (or "limp in").
Either your hand is good enough to raise with, or you should be folding it. This applies to the small blind as well.
Exceptions are of course if you are the Big Blind and don't have much of anything, you can still take your free check. Or, if someone else raised and you would have raised around that amount, you may call. If you were going to go for a bigger raise, you should still re-raise.
Try it for yourselves, especially at online poker where donkeys catch stuff on the river they have no right catching all the time, raise or fold. You will see a LOT less flops but come out more profitable for it