Giving your opponents odds to call. If you put your opponent on a flush draw (9 outs), then you have to make sure that you don't give them the pot odds to make that call. In order to do this, you have to bet enough to make it wrong for him to call. Meaning, you can't give him more than 4 to 1 odds on his money. For example, if there's $100 in the pot and you bet $50, then you're giving him roughly 3 to 1 odds ($150 to $50) to make the call, assuming that you will push all in on the turn if the flush doesn't come. 3 to 1 odds to call versus 4 to 1 odds to hit his flush is not enough odds, and in the long run, he will lose money.
Pay a player off when they hit their draw. It's okay to underbet the pot, BUT when you do this, you absolutely cannot pay the player off if they hit their draw. If you intentionally give your opponent the odds to call your bet, maybe to create action or something, then when they hit, you have to fold.