1. Buy yourself a tray service cart to get your machine higher in the air, so you can crank up the speed, and not have to park the ball machine off the baseline to practice returns and hard hit volleys and groundstrokes.
I got mine at Sears http://www.sears.com/shc/s/p_10153_1...0070921x00003a
but you can find them both online and in stores.
2. Work on any shot that is now a weakness until it is a strength:
1.) Returning serve. Half of all points start with you returning a serve. Everyone practices their serve, but too few practice thier return.
2.) Volleys. Everyone hits tons of groundstrokes. But most hit a few volleys before a match or during a hitting session and that is it. Don't stand there waiting the ball to arrive. Practice poaches by aiming the ball as if it was a cross court return and go get it from your usual doubles position. Practice the first volley on a "serve and volley" (don't actually serve) by running in from the baseline and take it near the service line again and again. Practice getting to difficult to reach volleys hit away from you to learn how to get your body into position to volleying on the dead run.
3.) Practice hitting backhands (or forehands) to the corners on the dead run from a neutral position, then recovering again to hit the next one ... and the next one ... and the next one ...
3. Keep turning up the speed, spin and feed rate gradually during each session to condition your neuromuscular reflexes to get to difficult balls. (It's amazining how you can do this in an incremental way to own a shot, rather than relying on chance balls during a match or practice hit.)
Remember Agassi's crazed dad made young Andre hit a million balls a year (about 3,000/day) from his ball machine to get real good.
How crazy are you? How good do you want to be?