Secure front fork when mounting on roof rack
I take it you are talking about the process of loading the bike, not when the bike is up there and the car is in movement (since most bike racks have a kind of trough that keeps the front wheel aligned). 

What I have done on occasion is to loosen the handlebar and turn it backwards about 75 degrees relative to the fork. The handlebar is then more or less aligned with the front wheel. Re-tighten the handlebar in this position. Then a bungee or other elastic material can be used to fix the handlebar to the horizontal frame bar. 

This does mean using an Allen key in most cases, but I figure it is worth getting the key out.