I think if soldiers and marines have to extend their tours, they should be placed in an instructing role. For example:
If one unit has just finished its tour, but it becomes extended for another year, then they should become instructors for a fresh unit coming over. They should teach them how to patrol, how to spot IEDs, etc. But more importantly, they could teach them the finer points, points that are geographically specific "Watch this tall building on the corner, snipers frequently hide there" or "this intersection has been hit with IEDs 3 times so far".