This is a really great idea for your garage.  Usually you see the tennis ball hanging from a string and it just gets in the way when you are walking around an empty garage. Plus, it just looks tacky.  With this set-up, the parking ball is moved out of the way for your convenience.

Video shot with my iPhone 3GS

Just cut the string to an appropriate length.  Mount one end to the top of your garage door.  Use an eyelet to hang the ball from the needed spot in your garage.  Pretty simple… You can totally do it yourself…