Mark, Don't Measure
Holding trim in place and marking it is always more accurate than measuring, often faster and it eliminates mistakes. This is good advice for other types of carpentry work too, like siding, laying shingles and sometimes even framing.
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.