Interestingly enough, early in my development I took a lesson from Jerry Briesath (master instructor/stroke guru). While he basically worked on the stroke itself, I asked about rotation drills. He had me go back and work on 3 ball drills; with the caveat that you can have ball in hand to start, but all 3 shots must be easy, high percentage shots ("Jerry 3-ball"). When you can do 15/20 racks with 3 balls, then add a ball. Do each layout until you make it, but it doesn't count as a success unless the first attempt is successful (and making all the balls does not count unless all the shots are easy - its up to you and your conscience to decide - my conscience occasionally cheats like a dog). My rotation skills improved markedly (but of course my stroke improved markedly as well - take a lesson from Jerry at any cost; greatest instructor ever IMO).