The purpose of the Q. & A. is to raise questions, problems or concerns and to clear the air.  Rarely should you want to prevent people from participating.  There are times when you want to keep total control, e.g., in dealing with the media or a difficult audience, but even then you may want to give the participants a hearing.  For most business presentations, this is an opportunity to get out any differences while you have a chance to evaluate them and to incorporate or discard them.  Skillful handling of questions and concerns in front of the group may prevent undermining behind the scenes later.
 
Preparation

List every conceivable question and answer ahead of time.  Ask someone to drill you.  Hold some research in reserve, on a visual or in your head, then present it as if spontaneously during your Q. & A.

After the last point in the speech, the audience will need time to absorb your last idea and to review the speech.  Give them whatever time they need to relax and start thinking.  If something isn't forthcoming after a suitable wait, you can begin to encourage people.   Suggest a question for them, "Sometimes I'm asked...."   Then answer your own question briefly.

Maintain eye contact and listen attentively to the end of the question.  This shows respect and acknowledges to questioners your support and concern for their point-of-view.  You will also be certain to understand the whole question and catch possible underlying assertions that need support or refutation.  Do not stop listening until the final punctuation.

Once the questioner is finished, take your time and begin the answer.  After the first few words, take the focus back out to the audience so that they are included again as part of the group.  This reasserts your control of the floor.

Keep your major premise foremost in your mind while answering questions so that you are not trapped into a tangential area, winning individual points but risking your original objective.

Try to keep your answers short.  If you don't understand the question, admit it, saying, "I'm sorry.  I don't understand the question."  Do not say, "Your question is unclear."   Rephrase questions that are unclear and rambling.  Repeat each question only if the audience cannot hear it.  If you don't know the answer, admit it.  Do not try to bluff the answer.

Try to move around the audience when accepting questions so that the total audience is involved. Before listeners will accept your argument, they  will want acknowledgment of their concerns.  Remember that evidence, not opinion, is what persuades an audience.

Make certain that you present the last word on the subject.  At the end of the session, make sure to have an appropriate closing:  a summary, request for action, request for commitment, or restatement of the group's agreement.  Remember, the first and the last comments made receive the audience's highest level of attention.