For me, I constantly read boards like this one and 2+2 (the two main forums I read, although I post much more here. 2+2er's yell "Get a rope" when a short stack strategy player shows up. )
I also read a lot of books on theory and strategy.
At the end of my sessions, (at least sometime in the evening before bed, often after dinner) I sit down with my partner, and we "replay" every one of my winning, and losing hands. (Using PokerTracker).
While replaying the hands we discuss them, and see if I made any mistakes, or if we notice any trends from the people I often play against. I also notice if I played the hand well, and make notes on the hands.
On thing I've most definitely noticed is.. when I first started playing, and got bad beat, I'd get upset and wonder how I could be beat so badly, so often.
Now, I sometimes still complain about getting sucked out on (Right IRC Gang?) but.. honestly, I play the next hand as coolly as the previous one. I pretty much shrug it off as "oh well, that's part of the game".
Bad beats have like zero effect on my gameplay now. I lose a buy-in, then re-buy-in, and go on.
As I have discussed with my partner, I think this is something that would have taken me MUCH longer to develop, in live games. The reason being, in live games you just don't see nearly as many situations, in the same amount of time.
If you're playing online, you're already seeing way more hands (likely double or more) than you would on a live table. Then multiply that by the number of tables you play, and you just see so many more situations in a given space of time than you would with live play.
By the time you get a hundred thousand hands behind you, I'd think if you don't have tilt controlled, that's something that needs to be worked on.