I've used various methods of dealing with this. Clip nuts are the best solution, but the plastic can be relatively thick in places and it may be hard to find a clip that fits.
Many times when the threads into the plastic are just stripped and the plastic not split or destroyed, you can coat the screw with Lithium grease or even car wax and then fill the stripped hole with super glue or epoxy. Lightly screw the screw into the glue, clamp and let it dry, you then have usable threads. Coating anything you don't want glued with grease (carefully) is a good thing to remember, it can come in really handy.