Basically follow general building/insulating practices.
Without seeing your garage, and knowing how much you want to spend, or where you live, it impossible to recommend anything.
I got a Kerosun brand heater (23,000 btu) for my garage. IT makes it nice and toasty after about 1 hour or so of operation in cold pennsylvania weather. However, you don't want to go this route if your constantly using oil-based chemicals such as stains and oil-based poly.