I pass work back and forward to a friend who does alot of hire and he holds a passport/driving licence as a bit of security. I think he also gets the client to check the equipment over and sign a form saying its in good knick, then checks it over with the client on return. I think he also lays out the replacement cost of whats being hired from the outset.
Sorry to go a bit off topic but i thought it was worth mentionig because heard of other hire companies loosing whole soundsystems and not being able to trace the client....