Watching a lot of game analyses.
If there was one thing that improved your chess the most, what was it?
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I would do this: read chesstactics.org. or another basic tactics book. Also read a basic endgame book, silmans will work. Now start playing games, play the longest ones you can get. And when you are finished, thoroughly annotate them yourself. This is all you should need for a while.
Combined with Chess.com you should look into using Chesstempo.com. The site is dedicated to improving chess players games. I use it every day, and if Chess.com already has a similar program, then I had no idea. Furthermore, I had the luxery of reading IM Jeremy Silman's "Complete Book of Chess Strategy" which improved my game signifigantly.
Do tactic problems daily.
Chessbase annotated GM games. It's much easier and faster to study, analyze games than ever before. Using physical boards and pieces is time consuming especially arranging and rearranging pieces. After playing a variation or subvariation I usually return to starting position again as I can't remember the original position before a variation 
Touch move.
I got to where I am today without playing more than a handful of long games (more than 20 mins per players)
Primarily i played 3 0 the first few years, and settled on 1 0.
Been playing for 4 years now, and I feel that if I have to make more progress, I'm going to have to put serious time and serious effort into the game, especially theorywise.
But the single most beneficial thing for me during these 4 years have been 2 breaks I took due to work.
When I was 1100 or so, I had to work 3 months in a small place without internet, so when I returned I saw my rating instantly jump to 1500 within less than 50 games or so. Then a year later, after fluctuating at 14-1600, I had another break, same length pretty much, and there i jumped to around 1800. I'm planning a 2 year long break, so when I see you again, I'll be playing with Carlsen and Kramnik..
I was serious about the breaks, perhaps the stuff I'd learned sort of simmered down into something practially usable subconsciously. Who knows.. I just know for a fact that taking some breaks from chess aparrently let me see the board in a new light when returning...
When you find out let me know. Sigh....
Analysing critical positions.
Reading games annotated for beginners, such as Logical Chess by Chernev. I would be a much better player if I could find even more games annotated for beginners!
AVStone wrote:
Getting really comfortable/quick with chess notation. Not only did it allow me to take in information from books and games more efficiently, but it also just gave me a better intuitive understanding/feel for the board and the pieces' movements. I would review master games to achieve this. Reviewing master games is great because instead of other things which only improve on a specific piece of your game, truely absorbing master games helps with all parts of your play (not to denounce other forms of chess study.) And of course, playing as many games as you can (preferably with a time control that allows time to think) also benefits your game.
Getting older.
kinkenwee wrote:
Losing. nuff said.
lol
One thing that has definitely helped me to improve is playing through annotated games of grandmasters and playing through the althernate lines given in the notes. One book in particular is my favorite: Rubinstein's Chess Masterpieces (100 games selected & annotated by Hans Kmoch). I also found Silman's Complete Endgame Course very helpful.
Endgame study.