We have an American Express Platinum card, which provides free secondary coverage. However, you can sign up for automatic primary coverage (with no deductible) for $29.99 per rental (which can be a deal depending on the length of the rental).

However, there is a funny workaround at most rental companies (we primarily use Alamo / National). You reserve with your AMEX card and the drive for your rental period. If damage occurs you pay with the AMEX and pay the $29.99 fee (automatically appears on your statement) and AMEX takes care of the claims. If no damage, just pay with a different card - they will let you switch your payment type when you are returning the vehicle (right on the handheld return units).

Primary coverage for essentially no cost.

We were in Honolulu in 2005 and rented from National. This was before the AMEX deal. I rented and declined coverage from the non-airport location because MN auto policies are required to cover rental cars. They gave us a full-size vehicle that had a dented/scrapped front fender. I really didn't care and left. Mid-week a criminal broken into the unit (breaking the driver's side lock) and stole a bunch of our stuff. We exchanged the car at HNL airport location and got a new full-size car with working locks.

National sent me a bill - not for the lock, but for the dent/scrap. I actually had saved my initial forms that noted the damage and the customer service representative "found" notations where the damage occured on a prior rental. So, they waived the charges. They never did send me a bill for the broken lock.