Very wise advice - thanks to the both of you. 

You can always trade pieces, but I try to force them out of the middle of the board with my pawns or bishops.  If a knight is on an outside row, you have cut in half the number of squares it can chose to move, because it would be off the board.  Other pieces are similarily weakened away from the center, but the knight is particularily weakened more than other pieces.

Another suggestion - don't play the studman!

i try to capture knights asap thats my defense