I use Black Magic Polish.  It shines up very nice.  Also about twice a year, I will hit the track of the ball with a 1500 pad to smooth out the track before polishing.   Most plastic balls do not flare very much so you can build up a noticeable track after a few months and that contributes to the "hook" you get in my opinion.

A few months ago, someone on here posted that they used rain-x on their plastic ball and it went "dead straight".  But remember rain-x is not "approved" by USBC for bowling balls. 
 

I have been throwing an Ebonite Maxim for about four or five weeks now. I am starting to notice it hooking off of spares late at night. I was wondering what the best polish/surface prep would be for plastic ball. I don't expect it to ever go perfectly straight, but I think it could be better. I read about snake oil on here, and wondered if it would work

Absolutely any bowling ball polish will work fine on Polyester/plastic balls. They don't have the same characteristics as resin or even urethanes; so there's no need to worry about types of polish with plastic. You just want as much skid and as little hook as possible. If you want to maximize the gloss, sand it as much as possible to remove any deep scratches, take it up to 4000 grit and then polish as hard as you like, using any polish that suits you. Heck, you can even use car wax & polish since plastic makes no difference whether or not the pores are clogged. (It makes a huge difference with resin; never use car wax on resin balls!!)

OK thanks! How often should I polish it?

That depends on what you see in the ball's reaction. I use mine fairly dull with wear; the ball still goes long and doesn't hook much and I'm rev dominant.

Polish as often as you feel you need to more length and less hook.

Like it can be proven that they used RainX, and why would they?

I may polish it before I start the winter league then. Thanks! Rainx eh? If I am correct, it is an oil that repels water. Perhaps it will also repel lane oil depending on its characteristics, allowing it to make less contact with the lane. Interesting. It is only against the rules if you get caught, and who will care about a person making a ball go STRAIGHTER. More people care about Joe Bowler using illegal methods to get more hook.

As has been said, there is NO reason to use RainX on ANY ball.

Well I do bowl in one of the few indoor bowling alleys where we can have rain delays lol

When did spare balls get so complicated?
Rainx? Soaking a ball in oil? 

Come on guys, hit the darn thing with polish and be done with it. No reason to get all complex.