My experience has been that the more carefully you put away the lights (to ensure the minimum of tangling, and jarring bulbs) the more strings that work the next year. The first thing I always try when a string fails is to push in along the line of bulbs, to try to re-set each them in the socket. It works about half the time. Modern lights are supposed to have a little dohickus that causes a "short circuit" when they die -- which allows the rest of the bulbs to light, but it's a touchy beast and must be properly positioned.

I really hate the cheap lights where the plastic work has been done so badly that you can't get the old lights or the new ones in without breaking them!