Cutting the inside and outside miters on the trim required us to place the wood 'finish side' down from time to time. Since the finished side was painted white, we kept getting marks on the painted surface from the aluminum bed of the miter saw. As you may know, aluminum is a dirty metal and will mark most things that rub against it.
We solved this by covering the surface of the miter saw deck with 'Frog Tape' and the problem of getting marks on the painted wood was solved.
Worked so well I thought I'd share. I'm sure any type of masking tape would work. We cut out along any moving parts of the miter saw and found that our angles and miters were not effected. I'm sure someone else has done this - but again - I just wanted to share it. Don't forget the supports on either end of the miter saw - if you have them.