What are some effective strategies for gaining an advantage in the middle game?

This is my interpretation of GM Larry Evans' guide to forming a plan in the middle-game:
GM Larry Evans' method of static analysis - Chess Forums - Chess.com
Read over my posts on the first couple of pages, and play over the sample games, paying attention to the notes to the moves.
I've always noticed that my opening is fine and I have good development, but I usually mess up with inaccuracies and blunders in the middle game, and lose alot of my games, what are some useful strategies I can use to gain an advantage?