The key is endgame. You can know all the fancy opening moves, complex defensive positions and postures for deploying your pieces, but if you don’t know how to progress toward, initiate, and conclude an endgame, you’ve got nothing, and your elaborate defense will crumble from the pressure. Either that, or, if you are good enough to thwart the opponent’s postures at endgame, you will result in a stalemate.
This is a lesson I’m currently learning. I can set up my board fairly well, and engage the enemy, but my endgame is lacking, and I often find myself in the reactionary position to the opponent’s endgame. Sometimes, I can maneuvar myself in such a way to thwart his attempt, but this ends in stalemate more often than checkmate of his king.
Also: Always take the initiative when you have the opportunity. If you are able to make moves in such a way as to keep the opponent reacting rather than initiating, you should eventually win. Obviously, if the player is better, or even matched, it may be a hard fought battle, or he may be able to outposition you and steal the initiative. But as a general rule, you should be directing the progress of the game, not reacting to your opponent’s direction.