In late August or early September, Jim Winterton wrote a post on Facebook about how to up your level of play by one division. I internalized the post rather than copying it out to a Notepad file, which turns out to be a mistake because now I can’t find it again on his timeline.

Basically, he mentioned two shots to practice to level up.
1) Drive serve, take at least 100 good drive serves per practice session, 3 or more times a week
2) Backhand ceiling ball, a few sets of ten each practice session

These two shots work both your forehand and your backhand as well as your ability to predict/anticipate/judge/move to the ball as it moves around the court.

Some videos to help with the drive serve portion of the drill:
Ben Croft demoing Drive Serves for Racquetball Warehouse
John Ellis demoing Drive Serves for Racquetball Warehouse
John Ellis demoing Drive Serves to the forehand side for Racquetball Warehouse
Jim Winterton demoing the Drive Serve with Racquetworld
How to Drive Serve demoed by Cliff Swain with Royster Racquetball
Jo Shattuck with Racquetball Academy demos including the drive serve