Novus #2 is my plastic polish of choice for light-moderate scratches. Heavier scratches get Novus #3, then followed by #2 (to remove #3's hazing).
I have Meguiars #'s 10 & 17, and they work fairly well on light scratches and minor hazing.
Like others posted, I only use some of my best microfiber and clean foam applicators.
Novus can be ordered online, or a local glass/plastic specialty shop may carry it. Got mine from a Regal Plastics for $5.00/bottle.
For a routine cleaning, Mother's Mag & Aluminum works extremely well. Excellent for removing yellowing and slight haze, but not really effective on scratches. I use this when just a cleaning is needed.
If the scratches are bad enough, wetsanding works well. I did my plastic headlights with excellent results. When doing them, I used Meg's DACP and Swirl Free Polish for my polishing steps. Both work well on plastics, but aren't quite as effective as a dedicated plastic polish like Novus.
Dave