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.
Don't Nail Too Close to Ends or Edges
Even with an 18-gauge trim nailer, you can split the molding if you're not careful. Avoid nailing less than 3/4 in. from the end of a trim piece or less than 1/4 in. from the edge.
Pin the Miter Before Nailing the Outside
In a perfect world, you could nail the trim flat to the wall and the miter would look great. But in reality, minor variations in level between the jamb and the wall often interfere. To solve this problem, start by pinning the inside edge of the trim, making sure the miter joint is pressed tight together. Then, while the miter is still tight, drive a pair of brads through the outside corners at opposite angles to pin it.