This Commerce Shipping advanced rules tutorial explores offering free shipping based on a total order value or based on a user's role.
Note that there are three parts:
1. Free shipping based on order value (the easy way)
2. Free shipping based on role
3. Free shipping based on order value (comprehensive, fancy, maybe too complicated. Don't watch the third one if you aren't ready for it :-)
The article that goes with this screencast is at http://www.commerceguys.com/resources/articles/248