Krylon Fusion Paint (black gloss)...It bonds with plastic and gives a great look.
I actually painted my windshield this weekend and it turned out fantastic.
Steps:
1: Remove windshield
2: Wash & Clean Windshield
3: Tape off the sides to avoid spashback spray and accidently painting the front.
4: Paint only the side that faces YOU the rider, do not pain the front.
5: Apply coats 8" or so away (you do not want to be too close) and avoid spraying an area to heavily to minimize the risk of pooling the paint.
6: Do some additional Coats and after allowing to dry, hold the shield up directly in light to see if you missed any spots.
7: enjoy!