As we play more and more poker, we of course gain more and more experience. Many of the mistakes we may have made in the past become corrected, and some of the reflections we have had in our past play become manifest in improved play. Thus, experience can be a great teacher, to the extent we take heed and seek to use it as a tool toward our future progress.
In order to best take advantage of this process though, it will become necessary to collect notes, describing our past experiences, of while we may use to reflect upon later.
You have a computer of course, or you wouldn't be reading this, so although some people may prefer an old fashioned pen and paper, it's best to take the notes on some form of word processor.
This may not seem like an original idea, and indeed it isn't, but you'd be surprised how few players, even good ones, use this at all, let alone effectively. And it is probably the single most important tool you have. Maybe even the most important by far, if you use it wisely. Set up a not taking facility, and use it well, and you can accelerate your learning curve dramatically. And there is always more to learn, regardless of how long you've been playing.
I like to keep the game types separate, which is a good idea. You'll want to sort out different types of games to be sure - limit, no limit, single table tournaments, multi-table tournaments, and so on. You also may want to sort out notes particular to a certain site, or even a certain game at a certain site. This is less important as making sure you take the right amount of notes though.
What you'll be commenting on is your own play for the most part. You can comment on the play of opponents in general or even specific opponents, but this must be kept separate. Primarily though, you'll be looking to catalog information on lessons you've learned about your own play - what you should have done differently in essence.
I'll give you an example. Let's say I'm playing limit, with a set, and I'm in a hand with an opponent who is re-raising. I look at the board and there's the possibility that I might be beaten, but he's got to have two perfect cards to fill 2 inside holes of a possible straight. Not much to worry about really I say, and he may be re-raising with a weaker hand than I have, so I re-raise him back. As it turns out, the player, who is loose and stupid usually, actually does have the straight, and I lose more in the hand than I would have if I had been playing less aggressively. So I make the following note:
If a better hand is possible from the board, and the opponent is betting like he has it, be careful not to get too aggressive.
I now have this note which I will be reviewing rather frequently, and this will get in my memory the error I made here, so that I may not make it again. I already lost more than I needed to on this hand, I reasoned that I should have played it differently, and decided that this advice would be sound. So I'm going to commit it to my notes.
Now, you may think that you've either played for long enough not to require such an exercise, or think you have a good enough memory that you wouldn't really need such a review, but you're mistaken in both cases. I've been playing for a very long time, and there is often ideas or thoughts that come up in my games, which I always make sure to write down for review later. And you may have a good memory, but it isn't perfect, but it will be with such a method as this.
The other aspect of this is to review your notes frequently. How often frequently is will be up to you, but it should be a regular part of your routine. Back when I was in college, I had a perfect GPA throughout my 4 years. This was at least in part from taking notes and ideas and reading them over a whole bunch of times. And despite the fact my memory isn't all that great, I always was fully prepared come exam time, in spite of going through nicotine withdrawal :)
Learning at poker isn't any different really. This stuff works very well if you're prepared to make the effort. Remember, you only get out of something what you put in.
As far as certain sites go, there are patterns of play which you may want to make note of as well. Some sites allow you to tag players with notes - this can be very helpful not only now but when you meet this player again in a future game. There is also software like Poker Tracker which will track your opponents' stats as well, providing the site you're playing on has the capability of emailing you hands. The easiest places to play usually don't, but it's not a disadvantage really.
When you're starting out this process, you may want to review your notes more often. Then, as you progress, you'll have to do it less often. As you're going through it, if there's a subject from the past that you feel you've mastered sufficiently, take it out of the regular notes and store it in an archive. You can review this archive occasionally to make sure the lessons stuck properly.
To sum up, note taking is an essential part of improving your game. Like all tools to improve, like these lessons, poker books and articles, and so on, they need to be reviewed regularly in order to have maximum effect.