Before giving the treat, get the dog in a sitting position. Then place the index finger of the other hand on his nose. Tap gently once and say "easy"- offering the treat with the other hand. If the dog lunches, withdraw the treat, and tap hard on the nose with a more loud "EASY". Dogs do not like to have their nose tapped (i.e. it emulates mommy dog biting them on the nose as pups). As you start to bring the treat back into range, if the dog even is beginning to act like it's going to come forward, tap the nose again gently, with "eh,easy". Once you have its full nose like attention it's much less likely to make a lunge for the other hand. If it should that calls for a very forceful tap on the nose, and a "NO". Using this technique, I've seen some "snappers" literally try to lick the treat from the fingers.