Just go up to them and say loudly... Fool get off your butt and dance with me!

Seriously, first prepare yourself for all kinds of silly answers and prepare to get 'rejected'.

Secondly, don't give up.

Before you begin asking for a dance look around the room and find several canidates. Start with #1 and if you get rejected simply move on to #2, then 3, then 4, etc... People come to dances to dance. It won't take you long to get a 'yes'; however, never expect everyone to say yes.

When you get asked to dance be prepared to say "Yes" or if you can't dance at the moment answer, "I can't at the moment, however, may I ask you later tonight? Then make sure you do go back and ask him later!