How do I get my puppy to stop biting people?

I've had my new puppy for about 3 weeks now and lately he's been getting really riled up and starts to bite people (not really hard or anything, but I just don't want him to bite when he's older - He's a Doberman X German Shepherd). He's also started to nip at the back of peoples' legs as they walk away from him. When you try to tell him not to bite, he just takes it as if you are playing with him and continues to do it and jump around aggressively while growling. Any tips/tricks that might help me? Thanks a lot.