Mick's solution would work in your scenario if you were to hang one tennis ball down on each side of the car, touching each side window at the very entrance of the garage (ie. just inside the garage door).
Start by parking the car centered exactly where you want it to enter the garage, hang the tennis balls so they touch the side windows, and voila! Perfect park every time.
You could go one step further and have another tennis ball centered on the front windshield at the end of the garage (as described earlier) so you have a destination point. Totally works... in my mind anyways!
