After developing your pieces and castling, you should focus on the following:
1. Increasing your central control,
2. Increasing your piece activity, and
3. Decreasing your opponent's piece activity.
These middle game guidelines are similar to the opening rules which you are probably familiar with such as occupying the center and developing your pieces. However, these ideas are applied differently later in the game. Controlling the center no longer means simply advancing a couple of pawns, it means mobilizing your entire force to control the central squares. Likewise, activate your pieces means more than simple development, it means to put them on the best possible squares. Finally, decreasing your opponent's piece activity means interfering with his/her plans to use the same strategy as you are using.
These abstract ideas are best illustrated with a game:
Unfortunately, I finished the tournament with two draws, so I tied for third. Nevertheless, there are important lessons to be learnt from this game. First, white played 10. e4, white followed the first principle, increase central control. Second, after black captured on e4, white did not instantly recapture and followed the third middlegame principle, restrict the opponent's pieces. Third, White played 16. Bg5, keeping black's bad piece on the board, also following the third principle. Finally, white played the move 21. d5. This is a critical idea which comes up a lot. After this move, white has succeeded in following all three principles. White's position is improved to the maximum, and the tactic starting with 23. Na4 allows white to score a quick win. Yet despite the seeming complexity of this game, all the moves were based on only three principles. These principles are the basis for much of chess strategy. You can also use these three principles to win games.
Happy Squeezing,
David Adelberg