This is assuming you play chess of course. I'm interested in what you GAGers have to say.
Most Helpful Girl
Most Helpful Guy
First of all, record all the moves while playing chess... they will help you realize what's wrong, when and where.
Next is opening... try to develop all your pieces during the opening. Also, try to control the middle 4/16 squares by when the middle game starts. Also try to create a solid pawn structure... avoid isolated, backward, doubled pawns (2 pawns in the same column) or pawn islands. Find a good opportunity to castle your king.
Now generally try to use tactical methods like pins, batteries, forks, zwischenzugs, sacrifices etc. I absolutely hate forks, they are very irritating.
In the endgame, try to promote your pawns as they are very valuable at that point of the game. Sometimes the king also can be brought to the center for attacking and also for hindering enemy king's movements.
I also watch matches Vishwanath Anand, Garry Kasparov and Magnus Carlsen to learn how they play.
Don't be short sighted when playing chess, analyze each and every move across the board.
Hope that helps