I am extremely careful now with noting all dents, dings, scratches, etc. Once Avis in VIE wanted to charge me $500 for a 1/2" scratch near the wheelwell. My credit card company reversed the charge as frivolous - I took a picture and you literally could hardly see it.

Another time a company wanted to charge me for scratches under the front bumper. Now I ALWAYS kneel down and look under the front bumper for damage and make sure it is noted (there is almost always damage there).

I also am sure to look at the roof and inside the doors - sometimes there is damage there.

And be sure the gas tank shows F. I have had more than one occasion where it has not been full.