In a league like Small World Hockey, its easy. You just sell him before his price drops too much and buy someone who is playing. In other leagues, hopefully you've got a good sub to back you up through the injury. All in all, I think you just have to be dedicated to the game, in RL and the fantasy league. Know your opponents, know what they've got, and know your secret weapons.
Sleepers are like the trenches in the middle of a battlefield, it's where the war is won and lost.
Those are the guys who give you the extra boost. Everyone and their mother gets Jagr, Kariya, Belfour, LeClair, and the host of other guys that are great. But only a select few can pull a point producing sleeper out of their hat. And maybe, just maybe, they'll be on your team. Good luck winning your TV's, cash prizes, and trips to the Stanley Cup.