Well, I'd suggest you start by trying to find someone who looks like he wants to dance. Someone standing near the dance floor tapping his feet/fingers to the beat is probably a better bet than someone sitting down on the side chatting with friends. As KShipper said, doing this yourself can also help increase your chances of being asked to dance. Then try the things others said -- go up to him, ask his name, do a little small-talk, and then you can ask him to dance (if he doesn't ask you first).
operabob has a good point that men can be self-concious, worried that the women won't enjoy dancing with them. And men do like to be asked, too. For one thing, it's probably the easiest way to know that a woman enjoys dancing with you. Most people won't turn down dance requests, if only to be polite. If someone never asks you, it's easy to think they don't enjoy it and are just being polite. If you ask them, you're telling them you enjoy dancing with them and that's good to know. :)