One of the best ways to ensure your garage loses less heat is by making quick trips in and out of your garage or using the garage door opener.
When selecting a heater, you should consider your garage's insulation. The more insulated your garage is, the easier it will be for your heater to bring it to the desired temperature.
If you have a garage with little to no insulation, you'll likely need a larger heater to overshoot the  BTU recommendations - and compensate for the added heat loss.