You have to get comfortable with the idea that you may not be able to answer everyone's questions -- but you can do your best to point them in the right direction.
Here are a few ways to say "I don't know."
1. Follow up later
"I don't have the answer to that right off the top of my head. Please give me your e-mail address after the presentation and I'll get back to you this week."
2. Share resources
"Here's my answer to that question, but I know someone who can answer it even better. I'll give you her contact information during the break."
3. Make use of present experts
"I see that Janie Expert is in the audience. Janie, I'd love to hear what you have to say about this question."
4. Ask the audience
"I have some thoughts on that topic, but I'm also wondering what others in the audience are thinking about this."
5. Give it back to the questioner
"How do you suppose you would handle that situation?" or "What has worked for you in the past?" Etc.
Please add your "I don't know" suggestions in the comments!