Dutasteride, known by the brand name Avodart. It's a more potent 5 alpha reductase inihibitor than Propecia and is the best thing we know of at maintaining current hair count. You should be able to maintain your current hair count for at least a few years on it, and then if you lose hair, it will probably be a very, very slow decline, only a fraction of the speed that the decline would be without it.