Touching audience members physically can occur at three times: Before, during and after a presentation.
Before the presentation many savvy presenters spend the entire time (after they have checked their equipment) shaking hands with audience members for the purpose of increasing their connection with the audience. Don't do the "politicians handshake" where you are looking for the next person to "attack" while you are shaking a person's hand.
Depending on the nature of the crowd, you might hug someone, or touch them on the shoulder. Watch out in "politically correct areas and businesses.
During the presentation you might have someone on stage with you. You may have to touch them to position them for a demonstration or put your arm around their shoulder while someone takes a picture. You also might go out into the crowd and shake someone's hand, or touch them on the shoulder while you are talking to them or other audience members.
After the presentation (if you were really good) people may want to come up to you and have their picture taken with you or actually hug you.
There are several issues at play and they change depending on your age, gender, demeanor and atmosphere. The first thing you don't want to do is send the wrong message with your touching. I have heard women complaining that a male presenter was just alittle too friendly. With women presenters, you probably don't want to cast an image of being a little too available.
Ideally the touching you do should be used to create a closer connection with the audience members so they listen more to your message. Here are some guidelines:
=> Be set up early so you can shake lots of hands before the presentation.
=> Don't concentrate on approaching only the physically attractive audience members.
=> Don't be rubbing necks and shoulders (unless that's what your presentation is about) on women. It could easily be construed as looking for "cheap feels."
=> Same as above. Be there early and shake lots of hands.
=> Be very careful of your attention to both the male and female audience members. Too much attention to the cute guys will have all the females and most of the guys against you.
=> Realize that you can get away with things that a guy couldn't. And use it to your advantage. An example of this is Jeanne Robertson who held male beauty contests and pulled up mens' pant legs on stage.