First, make sure you are using the correct fuse. If someone replaced it with a lower amperage fuse, and you keep replacing it with the same fuse, there might be a problem.
Second, and probably most important, look for a wire somewhere which has chafed against something and is vibrating or blowing around in such a way that the bare spot is hitting the metal frame of the bike. There is a good chance a wire got pinched under the gas tank sometime when it was removed for other service. Another good place to look is where the wire harness goes into the headlight shell.