I have to credit Tor Lowry for this drill. I think its one of the most useful drills to groove a straight stroke. Lowry recommends 2000 shots over the course of several practice sessions. 2000 isn’t necessary, but I ran my first century after doing that. It’s a great way to get your arm loosened up without having to worry about making a pot. The trick to this drill is that you have to do all the work in figuring out whether or not you hit the ball well or not yourself. Just slamming the balls in mindlessly will only ingrain bad habits, but if you really strive to feel how you hit the ball, how far you follow through, how straight your stroking, etc. You will find that this allows you to hit a lot of high-quality reps in a short period of time.
Use a bridge.
Shoot 1 rack with a tip of draw, 1 with left, right, follow ect.
Shoot 15 as soft as you can, or as hard as you can. Shoot every one just a little bit harder than the last, shoot them all off the rail, all with the bridge. The possibilities are endless, the key is to focus and hit lots of balls.
Pay attention to what part of the pocket the balls go into. If they are consistently a little left or right, you know your alignment or stroke is off.
Do the other stroke drills first to make sure you are stroking straight. This drill will ingrain a habit, make sure its a good one.
You get out of this drill what you put into it, a couple hours of heavy focus here will make you a better player.