If you'd like to get a slightly updated and higher tech solution to parking perfectly in your garage, as opposed to using a tennis ball on a string you might pick up one of these lasers. It will help guide you into your garage and make sure that you've pulled up as far as you can.
The laser will stay off until it detects a vehicle, at which point it will kick on and point a laser down at you. That way you know you're either not hogging one side of the garage or will know you can close the garage door behind you.
You can either purchase the Laser Parking System with one laser or two if you have a two car garage. For the single it will cost $24.95 and for the dual $29.95.