Check the Weather Stripping
The first thing you should do is make sure that your garage door is sealed correctly. Stand inside your garage with the lights off. If you can see any light coming through around the door or you notice any gaps, there's a problem. After all, if light can get in, then heat can get out. Inspect the weather stripping around the garage door and replace it if it's damaged or missing. You should also clean the weather stripping with an all-purpose cleaner to ensure it'll be as effective as possible.