The Rubber Band Trick - If you need to remove a nail, criss-cross a couple of rubber bands over the top of the hammer before using the hammer's claw. The rubber bands will protect the surface that you're removing the nail from.
A tried and true method. Cut scrap paper to the size of the frame and mark the nail location on the paper. Using painter's tape, move the paper templates around the wall until satisfied. Then hammer the nail through the marks. Remove the paper template and hang!