The Upgrade Travel Blog reminds us that some US domestic carriers, most notably United, still allow you get get a voucher when a fare you’ve already purchased goes down.
American used to do it domestically but no longer does, unfortunately. The piece reports that USAirways offers this feature.
What I hadn’t realized (I admit, somewhat embarassingly) is that United will let you go through the process online — a huge benefit for people like me who hate the telephone.