When on any trail or MUP, I wear a shrill survival whistle on a lanyard around my neck and blow it to let people know I am approaching, if they seem unaware and I cannot pass safely. The sticky wicket with this method is the number of people wearing iPods and the like who cannot hear anything in their environment. I don't have a lot of sympathy for a group trying to do a 25 mph paceline on our local MUPs; rarely is the other bicycle or pedestrian traffic so low as to enable this to be done safely.