If one's goal is to have fun or practice for OTB tournaments you may take a different approach, but if your goal is to win more games and improve results then it will be necessary to thoroughly research openings. You don’t necessarily have to do this before the game starts. You can wait until you know what options are available, but one thing you should avoid is just blindly wondering down an opening line only to find out too late that it has resulted in an inferior position. Know where you are headed before you start down the path.
In CC play at the master level it is necessary to know your openings and select them carefully. Not playing at the master level? Imitation is a good way to learn. Plus if you are playing at a lower level, opening study should produce even greater dividends. By opening study, I am NOT talking about memorizing reams of variations, but rather understanding the ideas behind them and resulting P-structures and tactics, etc. Of course one needs to know how to play the middle-game and endings, but all things being equal, an opening advantage gives you a head start in the effort to win the game. Besides, in this day of engine use in CC play, starting at even a slight disadvantage can be disastrous! This is far more true in CC play than in OTB play…unless you are a GM, of course.
One point…detailed openings study is NOT the best way to make yourself a better player, but it is necessary in CC play. Deep theoretical study is not essential to play at a high level, but you DO need a really good database of high level games. You cannot rely on the opening book that comes with Fritz or other engines. I recommend putting together a database of correspondence game played by masters because then you know they are well-played and the openings have most likely been engine checked.
When you select your opening line, look for wins by the highest rated players who played that line. That puts the onus of finding an improvement on your opponent.
If you do not prepare much before the game starts you will have to do a lot of work during the opening because you will need to review all the games you can find, survey opening theory, and you may have to try and find new ideas yourself.
During the game note keeping is also essential. One thing you don’t want to do is spend time rediscovering moves. Another thing I strongly advise against if you are playing on a site that allows using engines is that you DO NOT save the analysis then blindly follow it to the end! Once you have selected your move, delete the subsequent moves and if you want, type in the analysis as a note.
Also, when entering your selected move on the server, ALWAYS double check to make sure the move you are making is actually the position on the board and not from a position you are analyzing! I have both won and lost games because I was analyzing with a chess program, decided on my move then went to the actual position on the server and entered the move only to find out too late that the move was from a position I was analyzing and not the actual position. That happened because I didn’t look and verify I had the position right. I’ve done it, my opponents have done it and you will do it if you aren’t careful. I once beat a guy who outrated me by 200 points when I played e5 attacking his N on f6 and he didn’t play …Nd7. Instead he played the move he would have played on the NEXT move…obviously he didn’t look at the position on the server.
Sound advice from veteran CC players is that you should always devote more time to your won positions than your lost ones. A mistake in a lost position won’t be a disaster but if you blunder in a won position, it will be very painful!
At the same time you should be aware of the difference between bad positions and lost ones. Be careful not to write off bad positions prematurely and fail to give them extra attention. You should devote special attention at critical points in any position, even in lost positions. In bad positions you should try to set hard problems for your opponent. When you are really sure that the game is lost then resign!
Don't be afraid to accept a draw in an equal position! Your thinking time is probably better spent on winning good positions and saving bad ones than on trying to find ways to win an equal one. One point worth making here is that you need to distinguish between truly drawn positions and positions that offer equal chances.
Endgame technique is very important, so don't underestimate tablebases. If used properly they can be very helpful. They will help you decided whether or not to steer for an ending.
Remember, even with engines, understanding their peculiarities is important because you can’t rely solely on their output. Human interaction, things like playing through their analysis, engine vs. engine matches from the given position, etc. are all necessary before selecting a move are vital, if time consuming techniques, that will bring better results than simply letting the engine think for a couple of minutes then playing whatever it suggests. It’s also more challenging and fun which is why we play CC anyway!
In many cases even when using an engine it all comes down to who understands the position better, you or your opponent.
As one veteran CC player so poetically put it, all engines are useful when it comes to whispering cunning moves in your ear but it is completely up to you, whether you let your engine automatically play without your "help" or if playing centaur style, trying to create a plan and checking the moves with your chess engine. Right or wrong - it was my plan, you can say after your loss.
He offered another hint: Rybka, for example, is sometimes very generous about sacrificing pawns and often it is correct with its evaluation BUT it also still suffers from a serious lack of endgame knowledge in some types of endgames, but not all. You have to understand how engines evaluate positions!