One of the most commonly-used (and best) ways of improving one's game is through forums like this. Posting hand histories, analyzing other peoples' HHs and discussing poker strategies and theories with other poker freaks is a great way of bringing your game to another level.
Basically, overall you want to (1) read lots from people who have more experience than you (books), (2) play lots to get more experience, (3) review your hands and identify your weak points (forums), and (4) discuss hands, strategies and theories with others (also forums).
Obviously an online poker forum
with players from around the world provides a perfect medium for poker discussions. I would be meeting up with a live poker study group as often as possible, but I don't know a single person who's as obsessed with the game as I am.
Anyways, post any and all hands that you had trouble with, or thought you could have played better, or even just interesting hands for analysis. You might not get feedback every time, but it helps tremendously when you do.