The Dollar Dance is quite popular around here and we tend to offer two different ways of doing it for our couples who want to do one.
Option 1: Slower songs (think prom shuffle sort of speeds) and ask the Best Man and Maid/Matron of Honor to act as "bouncers," collecting the money and kind of regulating how long people are dancing with the bride and groom.
Option 2: The Contest: I will say this one only works if the wedding party and guests are there to have a really good time. Same as option 1, but the entire wedding party is needed to go help get guests up to dance with the bride or groom. We normally do guys vs. girls. Which ever side wins, gets to choose what song the losers have to dance to. About halfway through the "losers song" we invite everyone else to come and join them to open the dance floor. In this method, you'll have to count to see who wins. I make it a point never to say how much is actually gathered by either side.