Use a Shim to Cut a Back Bevel
Cut a back bevel on miter joints that are open in front but touching at the back. To create a back-beveled cut on a standard miter saw, place a pencil under the molding. If you have a compound miter box, tilt the blade a degree or two to cut the back bevel.
Smash Protruding Drywall
Occasionally window and door jambs end up slightly recessed, which causes trouble when it comes time to install trim. Correct minor level differences by either bashing in or cutting out the drywall along the edge of the jamb. But be careful to avoid going beyond what will be covered by the trim. If the level difference is greater than about 3/16 in., nail thin strips of wood, called jamb extensions, to the jamb to bring it flush to the wall surface.
Use a Brad Gun for Best Results
It's hard to beat a nail gun for perfect miters, especially if you're not skilled with a hammer. Trim nail guns allow you to hold the moldings in perfect alignment while you pin them in place. If you can afford only one trim gun, buy one that shoots thin 18-gauge nails up to 2 in. long. Fifteen- and 16-gauge nailers are good where more strength is needed, such as for nailing jambs, but the thicker brads make larger, more conspicuous holes and can crack thin moldings. Use shorter brads to nail the molding to the jamb, and long brads along the outside edges.