In my first two weeks of online poker, I have become convinced that table selection is the most important factor in whether you win or lose. It is critically important that you play better poker than the average at the table, and because of the rake, you need to be better by a margin.
But the best outcome seems to be when there is at least one really bad player at the table. One bad player can dump chips in your direction much faster than a whole table full of mediocre players. The fish come in a few different varieties:
The calling station is the player that calls many bets with a bad hand. They will often draw to bad odds, for example calling with a small pair in the face of overcards hoping to hit trips or drawing to a gutshot straight. Making money out of the calling station is a simple matter of betting when you think you're in front - this can include second pair or even ace high depending on the situation.
The weak player is always afraid of losing to a better hand. If they have a pair of kings they will worry about a pair of aces. If they have top pair, they will fear two pair. The weak player will often fail to bet/raise with the best hand, allowing free cards to drawing players. A raise (from good position) on the flop may scare them away from betting out their winning hand on the turn and river. The weak player can also be persuaded to fold a good hand sometimes. Be careful of bluffing too much however, as you will look foolish if your bluffs are called down.
The obvious player only ever bets/raises with a good hand. This allows you to fold with confidence with mediocre hands and even decent hands that you might otherwise lose money with if they turn out to be second best. When the obvious player is also a calling station, you are onto a winner all around, as they will pay your winning hands and allow your losing hands out cheaply. The obvious player will never slow play a big hand.
The loose cannon is that crazy player who bets/raises at almost every opportunity. They love to bluff, and probably think they are a terribly tricky player. If you want to challenge the loose cannon you need a decent stack, as they will sometimes actually land a good hand which could cost you a lot. Sit to the right of the loose cannon if you can. This will allow you to reraise behind them, which will get you heads up against them as much as possible. You want play a bit tighter than the loose cannon, but still pretty loose - you will want to call them down with your second pair. I find that many players seem to tighten up against the loose cannon, playing only high cards and pairs - this is great for me, as I can get heads up against a pair of random cards with a decent hand. Just watch out for when those tight players hit their hands and be prepared to fold in multi-way pots. I have seen a loose cannon turn up at a table and dump 20 times the big bet in the space of about 15 minutes - this kind of play will increase your expected winnings considerably!