How much should I tip the movers?

I am moving about 20 miles to a new place. The movers (there will be 3 of them) have estimated that the job will take about 5.5 hours, including driving time. I am moving from a two-story to another two-story, but access at both places is easy (no stairs on entry) and parking is easy (no slope or heavy traffic). There is also no particularly heavy object to be moved (no pianos, for example).

If they do an OK job, how much should I tip each mover? Of course, if they do a fantastic job, I will tip them more.