What's the best way to heat my garage?

It's a unattached 2 door garage non insulated, and every winter it's been really cold in their so i couple years ago i started using my propane 30,000 btu heater and it would help some but not enough, so what would be a good way to get it warm up to a good working conditions for cheap, I know its not really cheap to do it but whats the cheapest way?