Cost: $10 and up for weatherstripping.
First thing is to make sure that there is weatherstripping installed around the entire door frame, and that it is intact, pliable, and seals out most of the air from the garage. You can use smoke to detect air leaks around the door, or an even easier way is to turn on the lights to the garage and turn them off on the inside portion of the door and see if any light comes through. Next, make sure that your threshold is sealing the bottom of the door properly. You can use a simple draft stopper if you do not want to go through the hassle of replacing the entire threshold. One last often overlooked project is to caulk the trim around the inside and outside portion of the door. Air can find a way through the inner construction of the door frame if this decorative trim is not sealed properly.