When the middle game is under way, I take a minute to determine what the imbalances are: is material even? if not, who has what? who has more space? who has a lead in development? are there any weak squares?
So I try to determine which of those things are an advantage to me and which ones are an advantage to my opponent. Then its just a matter of maximizing my advantages and minimizes my opponent's.
For example, in our last game, by move 7 you had a good space advantage on the queen side and were cramping me. I had to do something to minimize that advantage. My knight on c3 was all but worthless and there wasn't enough room for me to manuever it to get it to the weak square on c5. So I just sacrificed it, ended up getting three pawns in the process, which meant that we were technically still even in material. That took away your space advantage and left the queen side open for my pawns to take over that side. The major imbalance then was our difference in pawns and minor pieces. Everything after that was based on trying to push those pawns up the board while limiting the effectiveness of your minor pieces.
A book that I read that helped me a ton was "The Amateur's Mind" by Jeremy Silman. He explains how to determine what the imbalances are and how to exploit them.
Hope that helps.
I'm trying to improve, as all of us are, and have a question:
Do you have a list of questions you ask yourself after each ply? I know the basics, "what is the threat? Is my king in danger?, etc.", but I would like to know what other questions you may ask.
Thanks