Make sure your frame is "square" before "skinning" it
If a piece is twisted, attach a clamp to it and then push the clamp up or down to straighten it and THEN attach it to the next piece
If your frame "bows" out or in along the middle, push or pull it out so that it is flush as you are skinning it. It usually isn't necessary to be 100% flush, but if you have the time, ability, and patience to get it that way, you should probably do so. It will save you headaches in the future.
If you accidentally put a hole through your facing, you can usually cover it with some painters tape or gaff tape and then paint over it. From the audience, you'll never know the difference. Just make sure to use a tape that paint will sink into. If it is rubbery like duct tape, the paint won't sink in.