I have had really good luck with good old Kyrlon in the old days. I painted some dice on the wings of my Kougar way back in the early 90s with it. Plane lasted a good 10 years and the paint was just fine. Just cleaned REALLY good with acetone first.
More recently I've used Krylon Fusion. (http://www.krylon.com/products/fusion_for_plastic/
I even had a place on an open structure where it got dinged where I had a paint stripe. It was Ultracote, so it just dinged, and didn't break. A LITTLE heat, the ding came out just fine, and you couldn't even tell on the paint stripe where the ding was.
FWIW, I paint trim work on as I always seem to have a problem with smaller shapes/paint strips, etc coming off if I use covering.