The strength of adhesion of the paint onto the plastic depends on how you prepare your item.
Ideally, your piece needs to be lightly sanded, degreased, and primed accordingly to the surface. Sanding increases the surface contact of the paint, and the primer act as medium between the paint and substrate to really grip onto those tiny places, additionally with natural thermal cycles of heat and cold the paint will contract and expand at a different rate than the substrate it's on, and that makes the paint crack and pop off. The primer act as a sliding surface that'll be intimate to the paint, more so than the plastic, so they will expand and contract together at a closer, more forgiving rate.
That's as much as you can do. Manipulation will create shear forces, compression, friction to the paint, so you gotta have the best adhesive procedure done in the first place.
- Preparation: sand with a fine sandpaper, steelwool, etc.
- Cleaning: Use a cloth that don't release fibers (microfiber, etc) to get the debris out.
- Degreasing: clean with isopropyl alcohol* (after this step don't manipulate the piece with bare hands, use gloves).
- Priming: Apply a plastic primer in a couple of thin coats.
- Painting.
