When I got Leo from the SPCA - we also had major problems with the nipping or play biting. It took us about 3 months to solve it. I used the no such thing as a free lunch system. If he started biting I would say NO BITE. If he continued to do it I would either walk into another room and end the play session, or would give him a time out in his crate. He soon realized that play biting was not going to get attention. I also used chew toys for him to direct him to acceptable chewing such as a kong filled with kibble and peanut butter or a hollow steralized bone with cheese filled in the middle. A couple of great books that I found helpful in training dogs was Brian Kilcommons' "Good Owner, Great Pets" and Carol Lea Benjamin's "Mother Knows Best, the Natural Way to Train Your Dog". Both books have great advice and techniques for training.

And yes - get your dogs neutered. That is imperative.