I think it was Sheldon Brown who suggested the best way to deal with this is to wear out the back tire, then rotate the front tire to the back and replace the front. The theory is you always have a good tire on the front (more critical to control the bike), and you keep the front tire from dry-rotting while you replace all those back tires.

Makes sense to me, so I try to follow it most of the time. (Except when the front is really, really new -- like less than three months old.)