There are two kinds of paints you can use...
Fusion by Krylon (made to molecularly bond to plastics)
or black trim paint (made for trim)
Either of these, if applied properly should offer a long , chip free life. It's no different than the painted plastic bumpers on your car. It's all about the proper application. Sand with 320 (lightly) and thoroughly, sand with 600 to make it smoother and apply either one of the paints I just mentioned in a mist coat like I said before - these kinds of paints are not designed to require a primer. Use a primer however, if you plan to use regular auto paint (such as duplicolor, etc).
If you use a mist coat and let it tack up (10 minutes) and then another mist coat (let it tack again) you can then use a third and final coat or be over-paranoid and do a 3rd mist coat before the final. As long as you dont wait until the paint is completely dry in between coats and you dont re-coat while its still really wet, it should last for a long time.
Before painting and after sanding, wipe it down well with a towel and water, let it dry and then go over it with a tack cloth to ensure that nothing is going to interfere with the bond of the plastic trim and the paint.