You need one of these:
http://www.purelycustom.com/c-121-sa...-dont-mix.aspx

(it hangs from the inside of the garage door, so when it opens it hangs down in your face).

I've also heard of people putting construction cones and trash cans out to block the garage.

I solve this problem by not having a car....

Sounds like you were very lucky to be able to salvage his bike!!