Lots of good advice (especially the first reply - all good points), but I'm not keen on headset mics unless used appropriately.

Yes, it's useful to have a mic when you need to get down on the floor to sort things out when people are hopelessly confused, or to demonstrate something when that's easier than explaining it. However I've worked with too many callers who spend all their time on the floor (even joining in a dance sometimes) and are blissfully unaware of a problem developing elsewhere on the dance floor in the middle of a dance or in a walk though.

Also the dancers can find it disconcerting to have a disembodied voice voice shouting at them with no obvious source.