The trick to the ouch trick is you have to simulate how it would happen in nature. You do indeed have to make the high pitched OUCH! but then you have to part company (walk away) and ignore the dog for a brief period. This is the part that is often times left out. In nature when an older dog in the pack (or a wolf, in simpler terms)is playing with a puppy, if the puppy bites to hard (this also happens during same age play) the older dog will Yilp and then completely part company with the puppy, sometimes for days. However when it comes to domesticated pets, 5-10 minutes is often times long enough.
Another option is to check out “Pet Corrector” made by The Company Of Animals. I have used it many times in aggression work. It makes a very very loud hiss like a natural predator and quickly the dog associates the behavior with being stalked by a large snake or a big cat. They generally run about $14.99 at your nearest PETCO. *WARNING* do not turn the can upside down when you use it, it will spray freon. Also, the trick to it is, the dog can’t see you do it.