Meguiars #7 Show Car Glaze and a toothbrush works wonders for me. Secret is the brush though. Without it its really tough to get the wax out of the texture. Plus the glaze makes the plastic trim look like new.
I think it helps to dress the trim with something before you wax, it helps removing the wax from the trim alot. Peanut butter might help, but not the first thing I would use The dressings I use for trim usually are quite solvent based and will remove any wax, and hold up alot better than any other dressings. Swedish brand Lahega, or Autoglym Bumper care.
I had really good luck w/ Duragloss WE (Wax Eraser), agitate w/ a toothbrush then apply Duragloss RBD (Rubber & Flat Black Dressing), RBD is very long lasting and nice matte finish, bonus: I use it on my tires as well. As mentioned earlier, if you plan to use the same staining material, then either put dressing ahead of application or tape off.