I got so busy in studying the opening game and middle game that I spend very little time working on my end game. Then I was reading a chess book where the author stated that it all comes down to the end game and that's where you need to concentrate your efforts. I am a beginner so you may want to seek more serious counsel from one of the better players here, but this is what I started doing:
I realized that, in the opening game, you were seeking to gain a strong control of the central square, plain and simple. You need to avoid wasted moves (for example, the Bishop going back and forth because you brought him out too earl). You need to build up threats to your opponent's pieces. Then you move into the middle game. This is where you are really starting to exchange pieces and clear the board for the end game. The end game is going to be a few pieces that can drive your opponent's King into Checkmate. You have to think about which pieces will work well together to achieve this goal. This means you have to consider their protection throughout the entire game.
I use Chessmater 10th Edition ( I also have and use the 8th and 9th editions on my other home computers and they work just as well) because they have a great built in training program. I suggest training software that give you chess problems to solve. However, rather than just fly through the problems, analyze them on a real chess board, try variations, and really study each problem in depth. Get books as well. You can google free chess ebooks and download a few great ones free of charge!
I have very little experience in this part of chess because I usually lose long before we get into what is considered an end game. for me, the end can come rather quickly.