This one tends to come up every time. Have some questions prepared.

This will show you have done some research and are eager to know and learn as much as possible. You probably don't want to ask more than three or four questions. Try and use questions that focus on you becoming an asset to the company. A generic question might be "how soon can I start if I were to get the job." Another idea is to ask what you would be working on and how quickly they expect you to be able to be productive. Remember to ask about next steps and when you can expect to hear back.