To win more in poker, be it free online poker or big cash you need to keep your opponents off balance , to unnerve the, make them fearful of your position and intentions, above all, make them pay through the nose for any transient gains or information you have to give them and in doing so allow you to set them up for even bigger falls later.
Your aim is to make them believe that the cost they have to pay for their potential gain in playing against you is too risky to justify so that they fold out or make errors and / or if they do go through with it and win a hand the price they pay is to give you information on their playing style while all they get is fearful knowledge of your unpredictable playing style.
Lets look at some examples of this in action.
If you have AA and the Flop comes 9-8-3, if you bet and you get a call, you expect your opponent to have paired one of his hole cards. Your opponent may also have a Straight draw with J-10 or 7-6. In the first case, you can afford to reach showdown because your opponent is also willing to reach showdown, but is unwilling to raise or call raises, so you play semi-aggressively; check-call or value-bet. In the second case, your hand may be unplayable if your opponent hits a Five or a Queen later. In this situation you have to play your Aces more aggressively than in the first case.
How is this?
When your opponent doesn't have anything, why should you play more strongly? Exactly - because your opponent doesn't have anything. In the first case, your opponent has a "something", but it is not strong enough against yours. You can afford a few value-bets because your opponent may fold if you bet too strongly. In the second case, you play more strongly because you want your opponent to stay in the state where he doesn't have something. If he hits the Five or a Queen (or a Flush card), your Aces are quite unplayable if he plays-back too strongly later.
Make your opponents pay for their draws. Usually they are unwilling to pay; if you check, they check too. If you bet, they call. So: if you think your opponent is drawing, don't check, and bet massively. Make the pot odds turn against their favor.
If they decide to call your bet, it's time to be careful. If they suddenly take the lead during the later rounds by betting or by raising you, you have to think twice whether they are really on a draw or not during the Flop. But if they check, take the lead again. Perhaps they are still on a draw, and if they have hit it or not, they'll remind you with their sudden and swift but all-too-readable actions, and you can fold your Aces without much remorse.
In the river, if no Straight or Flush-possible cards fall, you should bet again. Avoid showing your hand as much as possible. Your opponent can't call here - and will fold. If you show your hand then your opponent will get a hint as to what your playing style is. So make your opponent pay for this information as well.