Two answers:

1) On the practice side, work through books of chess puzzles. Set the position up on a board, don't solve it from the diagram. And write down your entire solution, including variations, before you check your answer against the book. I love the Yusupov "Build Up Your Chess / Boost Your Chess" books for this; the Chess School books are also great collections of hundreds of problems, but are not as didactic.

2) On the theory side, Dan Heisman's book The Improving Chess Thinker is an interesting read that shows how players of different abilities calculate and evaluate over the board. Seeing how players who are just a bit stronger than you think about positions is likely to be instructive.

This topic has come up so many times in so many places I thought about not posting about it here-but let's give it a shot. It is probably worthwhile because I hear so many people saying that these two problems exist for them (although they may call it something different).

In my OTB chess, I get good positions out of good openings against higher rated players most of the time. I often have a decent positional understanding of the game. My endgame technique is OK for my level (Class B USCF). AT TIMES, my tactical vision is pretty good although not good enough to take me to 2000 I think. But lately I am losing the vast majority of my games against higher rated opponents and some against lower and equal rated opponents.

This past weekend I lost 40 points USCF due to terrible play.  I am sure the problems I am experiencing are due to this issue.   

I am frustrated not because I dont know what my problems are, but because I know what they are and can't seem to fix the problem. My problems are as follows:

1. I have problems concentrating but I think it is a question of discipline.

2. My calculation is not broad enough or deep enough. In other words, not only do I not consider enough candidate moves, I also dont look very deeply into the position. I know I should, but I find it hard to orient my mind toward the process. I wind up playing and enjoying tactical games as it turns out so this problem is magnified.

3. I need to develop greater tactical vision.

The last of these I understand how to remedy. The first two I am failing at. I have even created a cheat sheet thinking process but just cannot stick to it. I dont think it is physical (tiredness, mental, etc). I think it is discipline and muscle reflex so to speak.

How did you train your muscle in this regard.

Looking for help (lost three out of three this weekend).

Thanks.