Book on Chess Tactics.

Look over your games and find out why you lost. Basically just look for better moves for you and your opponent. You can use a computer program to do this also, but its better if you do it yourself and then compare with the computer analysis.

All good suggestions, however, there is one problem the books don't cover. When reading the book, you know a combination is in the position, and often, what kind because the book tells you. You don't know that in your own games.....

you have to know the signposts that will tell you if there may be a combination lurking somewhere in the position. If you can't recognize the possibilty, you won't look for it.

There is one book which i read about that threw in positions which don't have any tactics just to keep you honest. I'm going to try to get my hands on that one.

Look for loose kings, hanging pieces, space advantage, development of both sides, etc.... Usually a tactic would be there if those factors are there.

"Can You be a Tactical Chess Genious?"

This is an actual book that I took out of my local library for a while. Most of it was a bit over my head but I think it had a whole lot of good stiff in it.