This advice is geared toward people just learning how to play correspondence chess online, though others may find these steps useful. This advice is especially important if you are struggling to get your rating above 1200.
Step 1: Don't play too many games or play too fast.
Online chess (or correspondence chess) is meant to be played slow. I know from experience that playing too fast and too many games at once leads to bad decisions and lost games even against weaker opponents.
Step 2: Learn an opening.
Use "explore games" to learn an opening. Most of the people under 1200 can be beat simply by knowing an opening better than they do. Get to know the ins and outs of possible responses and how to punish anybody who makes a bad move.
Step 3: Use the "Analyze" feature before every move.
On the right side of a game click "moves" and then click "analyze". This gives you a chess board setup in your current position to experiment with possible moves and your opponent's possible responses. Some people might argue that using the analyze feature might weaken your ability to visualize, but when you are under 1200, you probably are making tons of mistakes that would be prevented by simply double checking your moves. I believe your ability to visualize will come with simply seeing enough chess.
Step 4: Move with a purpose.
Every move needs to have a purpose. Whether you are attacking or defending, make sure that each move helps you accomplish a goal of attacking or defending. When you look over games from people under 1200, they tend to meander and the moves seem to lack direction, force, and tension. If you don't see a good move, pull out the analyzer and keep looking. You aren't always going to make the right move, but making moves that don't attack or defend will leave you wondering why you lost.
Step 5: Bring a bazooka.
It's amazing how few people really know how to achieve a checkmate when they start out. Turns out that finishing a game is harder than it looks. Bring multiple pieces into an attack and learn how to turn an attack into a victory. I've watched a number of games now where only one or two pieces are brought into an attack and the attack is not only fended off, but the game is lost once initiative is lost for the weaker player. Don't bring a knife to a gun fight.
Step 6: Practice.
Use Chess.com's "tactics trainer" 10 minutes a day to improve your ability to visualize and to get familiar with tactics as a whole. I've suggested that you use the game analyzer as you are getting started, but you'll be able to find the best moves faster and avoid traps if you use do chess puzzles 10 minutes every day. I personally use the Shredder IPhone application to practice tactics every day wherever I go -- but no matter where you find puzzles to solve, it should help your skills at visualization and at understanding tactics.
This blog post is geared toward new players. Not that I am qualified to give advice -- I myself am not an advanced player. I merely hope to help struggling beginners.