I hate painted interiors.

But I'll help you anyway.

If they are already poorly painted, forget about it. Buy new trim peices and start from there.

Wipe the new peices with a heavy degreaser... think acetone. Check on the back first to make sure your choice of degreaser doesn't melt the plastic, that would be bad. Then, find a plastic primer, I use some rustoleum product when I touch up exterior plastic (be sure to follow the directions on the label!) That will help the paint adhere to the plastic much better then it normally would. Then paint however you like.

And, moved to Appearance/Cosmetic.