Some things about poker are so obvious that once you know them, it seems you always knew them, but they are actually products of experience, so you didn't know them when you started. I'm going to talk about a few in the hope it will be slightly enlightening.
The nature of poker
At first glance, poker seems like chess with a bit of randomness, or bridge, or even backgammon, but it is not. In chess, both players have complete information (can see other guy's pieces and his moves can clearly be interpreted; in bridge, all the luck is at the start, in the deal, and can be easily overcome by skill, so that skill is the predominant factor (indeed, the point of bridge is that the deal simply sets the initial conditions for the game and from that point, luck doesn't enter into it); backgammon has a random element but also has complete information--it involves short-term luck but not so much that skill doesn't quickly prevail.
But poker is different. It's a game of luck that you use skill to navigate. You don't just learn how to play and win every time you meet weaker opponents, as you can in chess or even in bridge. Your skill is simply a way to make luck work for you. (By "luck", we mean simply "random events that favour you" in this instance.) I liken skill to the air in a lifejacket, which you wear while tossed by the seas of luck.
One problem players have as they learn--and I suffer from this--is that they mistake what they are learning to do. You are learning to inflate your lifejacket, not to smooth the waves. You are still going to be rising and falling with the waves of luck. And sometimes when you're in the trough, you'll be looking up at guys with barely any air in their lifejackets, who are nonetheless enjoying the peaks.
I find that hard to bear, because one thing I have always liked in games is that they reward skill promptly. You bother to learn them and they repay you. They have an internal justice. But poker doesn't, at least not in the short term.
Most learning in poker is learning about losing.
I'll briefly touch on this because it's important to realise that most of what you learn in poker is not how to win, how to overcome villains or how to make the max, although some of it is. No, you mostly learn how not to lose. One of the first things you learn is to fold a lot. You do this because not folding loses you money. You might then learn not to call. Partly, this is because calling can't make anyone fold, removing an avenue of winning, but mostly it's because calling so often loses money. And the question you are often asking yourself is not "will I win if I call here?" but "will I lose?"
You do not know whether what you will do will make you win.
In chess, you can figure out your opponent's moves in advance, and think up countermoves. You can work out strategies that you can be sure--be you skilled enough--will win.
In poker, you can't. You can't see the other guy's cards. This has the curious outcome that even though all spots you are in have outcomes that either favour you or don't, because you do not know what he has, you must find another means of deciding on your strategy.
Good players realise this, and instead of "putting him on a hand", they "put him on a range". It sounds more complicated than it is. If a guy raises 10% of his hands and he has raised in this hand, you can assume he will have a top 10% hand. This may not be super accurate, because he might raise all the top 8% and some bits and pieces for deception, but it's as good as you're likely to get.
The flop comes and now you do not know whether it hit him or what he has. But you can figure out what he would have for each of the hands in his range, and then average it out to see how you stand. Of course, we don't do that at the table. We just think "did that flop likely hit his range?" most of the time. But sometimes--often--we consider our hand, which we know, with his range, which we can figure out, so that our play will be correct against the range, if not the hand he actually holds.
This is the best you can do in poker. The principle is that you will be in the same or similar spots many times, and the best you can do across those spots is to make the play that maximises your outcome on average.
So the guy raises, the flop comes 853 and you hold 99, so you think you may be ahead of him. His range has more unpaired overcards than pairs (I'll explain how you know this another time), so on average you are winning. He bets, you raise and...
Well, obviously, if he has AK or AQ, he folds. Nice. But if he has KK, oh dear. One outcome is wonderful; one horrible, and you can ever know which you are going to get. You will wish you had known when he shows KK!
It gets worse. Say you are considering pushing a hand, say AQ. You think the other guy will call with 20% of his hands and you are taking it that he will call the top 20%. This is an approximation, of course, because most players do not know what the top 20% does or doesn't include.
So sometimes you will get it wrong because he'll call with J7 (not top 20%), making your shove bad, and sometimes you will get it right because he'll call with AK, but oh shit, that's towards the top end of his range, and your play, good against the range, is horrible against AK.
It's almost never worth chasing.
"Chasing" is calling bets trying to hit a draw. It's not to be confused with betting a draw. In most forms of poker, particularly in cash games, you can call bets when you have the odds to do so.
What? Well, remember, we are trying to make the plays that have positive EV. If you are offered 3 to 1 by the pot and your hand is 2 to 1 to make, you will make money by calling the bet. Two out of three times you will miss your draw, but the other time you will hit and win enough money to make up for the times you miss.
No limit complicates matters because you can often get higher "implied odds" (IOW, the pot may offer you 3 to 1, but that's not all you hope to win because there will be further betting, which you hope to have the best of).
But two things should be kept firmly in mind. Effective odds--the odds you can get now--are fixed and can be counted on. Implied odds--even the improvement in your odds brought by a call from someone you are certain will call--cannot be counted on. Sometimes the other guy just doesn't pay off your set or your flush.
Setmining is the key example of a spot where you hope to have implied odds. It's t20 and some guy raises to 60. You hold a pair and call. You will hit your set about one in nine times, so you are definitely not getting the odds for your call. But you hope to be "paid off" enough the times you do to make up for all the times you don't.
If the other guy is tight though, you may not. Most raising ranges have more unpaired than paired cards, and most times the other guy has nothing after the flop. So he might cbet, and you can take that, but a tight player just won't keep throwing money your way, particularly if he realises you are also tight.
So you have to be realistic about implied odds. And on the whole that means, particulary in STTs, that you shouldn't chase draws. You'll end up regretting it, particularly because hitting doesn't improve your position as much as you hoped all that often.
This can be the one time.
Fish chase all the time. We don't mind that they do. (We only mind their hitting!) We offer them bad odds and let them try to beat us. They like to chase because they remember the time they stacked some guy with a flush, and don't remember all the missed draws (see "Salience is your enemy" below).
But when they are 4 to 1 to hit a draw, you should not forget that this can be the one. And it hurts when it is. You forget all the times they didn't get there (largely because you do not know for sure that they were calling with a draw until they hit it). But each time, this can be the one time.
Remembering this really helps you stop tilting when you lose longshot bets. It does all work out but that means that you have to lose some. A 70/30 seems like a really strong shot for you: KK vs AJ for instance. But you have to lose the 30 to win the 70, and that's a lot of losses.
Salience is your enemy.
Say you have a boyfriend. You see him every day and you get on well most of the time. Most days you meet up, have dinner and fuck, and it's all cool. But every now and then, you have a big row.
Sometimes people split up because of the big row. They forget all the cool nights and remember only the fight. That's salience. In general terms, change is salient for humans.
In poker, the times you win are not very salient, because you expect to win if you are a good player. What sticks are the times you lose horribly. For bad players, it's pretty much the other way round. They remember how great it was to have a flush, so they call bets with their sooooooted cards and chase their flushes every time.
Either way, salience is your enemy. You need to learn to play hand by hand. I remember a great post by Gigabet in which he stressed this approach, and it's only recently I've realised what he meant by it. Abandon expectations and play the hand as it is.
Actually, this is great advice for the whole of poker, and I wish I could follow it: abandon expectations and just play. You have to trust yourself to do the right thing and let the results take care of themselves.