I really like this "Best Answer" method of training a dog to know about using a soft mouth, learn "Gentle" while learning how to use impulse control with their mouths. This method makes them have to figure out what works over and over and control themselves to produce the gentleness it takes to get the treat. Thereafter when a dog starts playing/biting too hard, you can always "Gentle" them into controlled behavior using a soft mouth. Be careful though when training the dog that biting hurts as some dogs get excited & positively love it when you cry out much so the simple word "Ouch" is better in case your dog is one of those.
Playful dog bites to hard? - Yahoo! Answers
A good training program will help your dog learn impulse control and how to love listening to and responding to your commands.