After you approach the mic, don't preview your eagerly-awaited blog post detailing your brilliant theories on the burgeoning importance of the augmented reality biometrics space or whatever. We don't care. Just ask your question.
Unless explicitly asked to introduce yourself before asking your question, don't. We don't want to know about your blog or your startup or your dissertation. (If we do, we'll come find you after the panel.) If asked to introduce yourself, name and affiliation will suffice.
There's no need to preface your question with, "I'd like to ask a question about the use of Twitter in classroom settings." If you've gotten to the mic, you obvs have permission to ask your question. So ask it.
Consider this simple formula:
Awesomeness of response /
Length of question =
Quality of question.
The single variable in this equation you can really control is the length of your question. You are aiming for the shortest possible question that produces the most thoughtful possible response. Be economical.
If you're about to ask a yes-or-no question, reconsider. Warning signs that you might be about to ask a Y/N question: It begins with the words "is," "do," "will" or the like. Try reframing it with words like "how" or "what" or "why."
Staying at the mike (or standing up) encourages you to think you're in dialogue with the panelists. You're not. If you have a truly awesome follow-up question, go back up and wait your turn in line again.
Keep this single principle in mind, and the other stuff should follow.