I posted a thread on the same thing not long ago. Gretchen was biting and being obnoxious even with lots of exercise. Here's what we learned:
1. In the afternoon she was overtired and needed to be crated to enforce rest or she would become the land shark equivalent of jaws. A tired puppy is a happy puppy. An overtired, overstimulated puppy is a nightmare. Crates are their (and your) best friend.
2. She needs something she's allowed to use her mouth on as roughly as she wants while playing. We have tug toys she is only allowed to play with with a person. Bones, chew toys, etc. did not fill this need. When she gets a little mouthy, my kids will say no and grab one of the tug toys (we have them stashed around the house for easy access). This has virtually eliminated land sharking as she has an approved outlet for those desires. I've answered the door for Fedex with Gretchen dangling from a tug...
3. Her mind, not just her body, needs to be tired. If I don't come up with cool things to teach her, she will come up with activities on her own and I guarantee you whatever it is won't be something desirable. I am not a trainer by nature so I have bought a couple leerburg videos and some trick books. It builds a positive relationship between us. I'm really enjoying it. Sometimes I wonder who is training who, but I think that is part of owning a GSD.
4. Basic gestures of respect. Before she eats, when she greets new people, when she's presented a toy, when I open the van door, etc., she is expected to sit politely and wait for my instructions. When in doubt about what to do in a given situation, she'll turn to face me, sit, and give me her full attention. It gives her power in that she knows this is guaranteed not to be a displeasing act. It may not be what I want, but it gives me the chance (since I have her full attention) to show her what I want.
ETA: Gretchen is 18 weeks now. At 12 weeks her biting was so bad I thought buying her was a huge mistake and that she would have to go back to the breeder. Thanks to the people here, I realized *I* was the problem. The above is what we learned she needed to be a happy, balanced dog.
This is what we have today. That is cereal my daughter is eating, and Gretchen loves milk and knows what is in the bowl.