Move the front tire to the rear when the rear tire wears out and put the new tire on the front.
This is a ritual that I go through every 1,200 to 1,500 miles with our tandems which, if you haven't thought about it, wear through rear tires twice as fast as single bikes. So, from an economic standpoint, it's the only way I can get an honest 2,400 - 3,000 miles out of a tire before it's shot -- assuming we don't cut a tire that SuperGlue can't fix. From a safety standpoint, it also ensures an "old tire" isn't left on the front just because the tread compound "looks" good. Oh, if a front tire gets a cut that can be fixed by a dab of SuperGlue it gets replaced once we're home and sits in queue for use on one of my single bikes or the back tire of the tandem once the current one wears down.