I bet that polish used for clearing foggy headlights might be a good choice. I'd personally just try any compound for plastics.
I have gone with a dremel tool buffer and compound before, and found it works awesome, but I needed a steady hand to make it buff evenly.
You can also paint your balltops. I've done it with awesome results. This is probably never recommended around here because:
A) It is a lot of work, and you have to know how to paint with some skill.
B) If you don't sand, paint, prime, and clear coat it with multiple coats, it will wear off after normal use.
C) A lot of times, it is cheaper and easier to just buy a new balltop.