If there was one thing that improved your chess the most, what was it?

It could be getting chess lessons or a chess.com membership, anything you could think of. I'm looking for something to improve my plaing strength.

Besides just playing a lot, maybe watching master chess tournaments and matches; trying to understand why they make the moves they do.

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.

So reviewing annotated games and playing lots of games are both good.

Having a stronger player guiding your progress.

Tactics, tactics, tactics.

Playng correspondence chess (Postal, now called online chess) the way it was meant to be played. Spending hours every move. Chess by postcard forces you to take your time. The quality of online chess today is poor, players often takes seconds, or minutes deciding on a move. It is not a substitute for over-the-board play, as many believe.

Anyway, playing postal chess forced me to analyze, and learn openings and endings. I never would have achieved a master title without the training it gave me. My free video lessons course on you tube teaches how it's done!

http://www.chess.com/groups/home/nm-aww-rats-free-video-lessons

Do you guys know of a correspondence chess organization That does seem like a good idea.

IMHO playing live chess is a good way to improve. Computers are ok and the blitz games on the internet are fun but there is no substitute for sitting down at a table and playing a real live opponent, face to face. Perhaps a situation like that evokes our primitive fighting instincts.  

I am also going to create a training regimen which involves those exercises.

Back in the mid-'70s, after two years of poor tournament results,  I realized how pathetic my tactical play and calculation truly were.  At the time there were no resources like our 'Tactical Training'  or the excellent videos on those topics,  but there were (still are!) good books.  I read Kotov, THINK LIKE A GRANDMASTER.  Never followed his method fully,  but he gave me basic ideas about how to calculate.  Then it was constant, daily work in solving tactical puzzles from books.

There are many topics/resources I continue to use in the lifelong quest to play better chess.  But tactical practice remains at the top of my list!

(Incidently,  I find the Tactical Trainer on this site most useful when I disable the 'Rated' function.  It puts too much emphasis on speed of calculation.)  

Those are good pieces of advice.

CCLA. Correspondence Chess League of America. Google it.

Thank you for the link!

Endgames.

What is the best endgame book? Is it Silman's course?

Chess mentor it helps a lot,TRY IT!

Years ago, I found a very useful endgame book - PANDOLFINI'S ENDGAME COURSE.  Covered the basic types very well.  And as Rifaram says, there's lots of good endgame material in the Chess Mentor lessons.  The videos too. (Your 'best buy'  may be to get yourself a premium membership here; compare that to your potential cost of buying books/videos, it may work for you.)  (That wuz an unpaid plug!)

Finding strong players in my area and buying them coffee.

Playing hundreds of correspondence games. That really opened up a whole new sphere of the game for me. But I'm still a beginner.