Audience participation is not to everyone's taste. Some members of your audience (including me) will get up and look for the exit if you try to get them involved in a group exercise, such as "turn round and tell the person next to you how good they look". Probably because I cringe at the thought of mass participation, I rarely include it in my presentations.
However, I do sometimes involve one or two audience members in simple exercises to make a strong point. If you want to get someone on stage, asking for a volunteer does not always work. Of course, you can ask someone in advance, or find a friend or colleague to work with. I prefer to ask someone I don't already know, because I think it makes the point more effectively.
Here's a technique I use. To begin with, I ask the audience for a show of hands on two or three topics. I then ask if anyone has a question, or an experience they would like to relate. There are always several people that are happy to engage in dialogue, providing the opportunity to build a relationship. When I need a "volunteer", I return to one of the people I spoke to earlier, and ask them if they could help me out for a minute or two. They always say yes.
One more thing - never humiliate or patronise (as if you would). You're not a stand-up comedian (OK, if you are, look away for a minute). Be respectful and polite. And give them a reward for taking part - such as your latest DVD (yes, it's an advertising opportunity too).