Little propane space heaters work great to initially heat up the area you are working in and to heat up the floor you are working on.
http://www.comfortchannel.com/images/SPC125KFH_HERO.jpg
But you don't want something like that to heat your garage at all times.
Good luck, and I am curious to see what other people suggest because I am in the market for a heating system for my storage garage.