There are products that you can get from paint supply stores that will grab onto plastics really well. I recommend Bulldog. Prep and spray the part according to the directions. If it is a rubbery plastic, you can also add it to your paint for increased flexibility and adhesion.

I think they also have some rattlecan products designed to promote adhesion, then you just use the paint of your choice on top.

I've used the Bulldog with good sucess on really flexible parts, with none of them coming back with any problems.