If you're going with a solid color, Krylon is now making a paint designed for plastic surfaces.
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Krylon® Fusion is the world's first no-prep superbond paint for plastic. This revolutionary product makes life simple, again. With no sanding or priming, this product dries in 15 minutes or less. Fusion bonds easily to plastic, PVC, hard vinyl, resin and more creating a durable, professional-looking finish that won't chip, peel or fade.
http://www.krylon.com/main/product_template.cfm?levelid=5&content=level
I'm a little old school, and don't exactly buy their no sanding theory for glossy plastic surfaces, but I could be wrong. A light surface prep via wet sanding with some fine grit would be my choice.
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A little OT, and not for helmet painting -- Krylon makes a latex based spray paint that is a pure joy to work with. Overspray is minimal, and a can goes a very very long way. The finish is slightly nubbly, but it lays on so evenly you almost can't go wrong.