Before booking, read the policies (rules). So far I just go by those on the company's web site and those on Travelocity .com, assuming the worse case applies. For example some companies require that you purchase their collision if you don't have collision on a car you own or you are a non-citizen. Beware of geographical restrictions. (Not absolutely foolproof, a location's policies might differ.)

Get and bring a written confirmation of your reservation.

Read the contract before you sign or initial. You may have told the agent you are declining a coverage but the "accept" space is circled and the agent points to that place to initial.

Before starting off, inspect the car for damage no matter how slight. If this helps make sure you don't miss a spot, drum your fingers all the way along the sides of the car as you look. Inspect the inside, too, for cigarette burns, etc.

Open the trunk and look for all the tire changing tools and spare tire.

Take your time doing the above.

More rental tips:
http://members.aol.com/ajaynejr/carrent.htm