Aggression!!!

Ohhhh yeah baby, now the fun begins! Good aggression is the key to no limit holdem, it is the hardest skill to develop and it is the one that will show the more profit! First, let me tell you what good aggression DOES NOT mean:

Good aggression does not mean trying to push everyone to fold with a weak starting hand
It does not mean bluffing on the turn and the river with Ace high all the time
It does not mean calling a lot!!!

Now that you know what it's not, let me tell you the basic lines of aggression:

Aggression is about making those continuation bets in order to steal many pots on the flop when other players haven't hit
Aggression is about betting the right amount on draws to push people with weak hands out and hide your draw
Aggression is about sometime reraising players that are betting the flop even though they have not hit
Aggression is about learning to "fold or raise" much more often instead of calling
Aggression is about playing less hands against a raise preflop but reraising with the premium hands

Calling less

I have not make any studies on the subject and can't bring you any proof, but any professional players would tell you I'm right when I say: at least 90% (and most probably like 95%) of online poker players would make more money if they would never ever call, always take a decision between fold or raise. That does not mean it's always wrong to call, what it does mean is that most of their calling decisions are mistakes. Why? You call when you think you might win the pot, but you are not sure enough to reraise or because you have a draw.

When can you call?

You can often call preflop, especially with small and medium pocket pairs, suited connectors and suited Aces in good position when there are limpers. Preflop, calling is often much less of a mistake than on the flop because you pay to see 3 cards. You still have to play tight, but your 66 has 3 chances of flopping a set preflop, but on the flop, you will pay more and have only 1 card to hit your set.

You can also call more often on the river. Not much more often, but a little. Why? Because there won't be any more betting, so if the guy bets 4$ in a 10$ pot, you're paying 4$ to possibly win 14$, if your hand has enough chances to win, go ahead!

You can also call with draws when you have the right pot odds and you know a reraise will most probably get called. Most draws have less than 50% chances of winning the pot, so betting is good to steal the pot or hide your draw, but if the other player almost never folds, you should not since you will lose more often than you will win.

When to fold?

You expected the next part to be about raising? Well sorry guys, but learning to fold is just too important. Raising all the time is just not good aggression!

You have seen the preflop charts, you adjusted your game... good! But now, you have to learn to fold even more preflop! Fold more against a raise! "Trying" the flop is usually not a good thing against a strong player. If a strong player raise and you have AJ, unless the table is really shorthanded, you should most probably fold! You can lose too much when they have AK, AQ, JJ, QQ or KK and you will not make much if you hit your A or J vs something weaker. Against a strong player, you need to do much much "raise or fold", even preflop.

The flop and the turn are the worst betting rounds to call, so if another player bets and you hand is not good enough to reraise him, you should fold most of the time.

When to raise?

When you raised preflop, you should bet on the flop almost every time (see my article about continuation bet). The idea is to correctly size the bet! How do you size a bet? There are many factors, and here they are:

The probability that you will take down the pot right there! If a bet of half the pot will get all your opponents to fold 75% of the time, do it all the time!!! If you bet the size of the pot, you need your opponent to fold at least half the time to profit from betting, if they don't fold that often, you should bet less. If you are unsure, consider that betting between 50% and 75% of the pot is usually the right spot!
The number of players still in the hand: the more players still in the hand, the less chances you have to take the pot right there, so don't bet the size of the pot in a big multiway pot with ace high! If there is more than 2 other players in the hand, you should consider checking if you don't have a strong hand.
The probability of being reraised: If your opponent tend to check-raise you often, you should bet less with a weaker hand, in order to lose less when they do. The idea is to make the bet profitable with all those times you steal the pot right there.
Possible draws: You have to bet more if there is a possible draws, other players paying too much to draw is a part of a professional poker player's winnings! If your bet is too small and your opponent's have pot odds, then you are the one to lose in the long run!
How you are feeling: Yes, I am serious! You can't play the same way all the time, some players will be able to read your hands. You raise preflop with AQs, you get 1 caller and the flop comes 249. You have a flush draw, your opponent checks and the pot is 10$, betting 5$, 6$, 7$ or 8$ would all be good decisions, do as you wish! You feel like trying to bet 6.87$, why not, try it! That's the way to play poker, you figure some kind of range of good decision and you go by how you feel, that way, your opponent won't know if you get top pair or a flush draw. Just remember that feeling is not a valid reason to justify a bad decision.

Now, you waited for a strong hand preflop, you made your raise, you did not hit the flop and made your continuation bet, but you got called again... If you still have Ace high on the turn, most of the time, you have to start being honest here and check. You don't have to play every single hand where you don't get nothing strong like that, but most of the time, you should not bet the turn or river with a hand that is most probably beat.

That is the general idea of aggression, if you haven't already read it, you should definitely read my section on continuation bet and draws.