I use Costco for car rentals. I do what Clark suggests and reserve a car well in advance of a trip, then check back periodically to see if the price has decreased. This works well, I have a trip coming up in April to Hawaii and made a reservation several months ago for an Intermediate SUV for ~$1000 for 12 days. By checking back periodically, I now have a reservation for the same time for $507. Clark says he checks back a week or two before the trip to see if a better deal exists, but I have been checking daily and have found that some times there are better prices that only last for a day. these could be missed by only checking weekly.
I currently have reservations for that trip for Intermediate and standard SUVs. I have the Standard SUV price at $625 now. That price was only available for one day, the previous reservation I had was for $679.