Just absolutely don't sand the plastic lenses before spraying them -- it'll ruin their optical quality and result in lousy light transmittance ! I applied a spray de-greaser, followed by plastic adhesion promoter, followed by coats of tint, and then clear coat, and finally wax (also some gentle wet sanding following the tint and clear coats). Am happy with the result, but it took time and patience!
I tried VHT Nightshades on a hitch light, the results were ok but not good enough for my taillights. I took them to a buddy that does custom painting and they turned out beautiful. He followed the process that TrevorS states. I'll post some pictures tomorrow.