For this year's trip, we got our best price just under two weeks out. We have a standard SUV reserved with Alamo for $285 taxes included. A full size car is running at $139 all-in, but we enjoy the larger vehicle as that is what we drive at home.
Until about two months out, check "regularly" (every week or two) just to keep an eye on the rates. Rebook if you find something lower. Never lock in early with a full paid/uncancellable reservation as prices WILL go lower before your travel date.
Once you are about 10 weeks out, look more often (at least once per week). At 8 weeks, start checking every few days. By 4-6 weeks, you may even start to watch more than once a day, as rates can often be very volatile and you may get in on a big drop before they vault back up.
There are usually car rental threads for each specific travel month on the Transportation board. Join the May group and then you'll have the added benefit of sharing in the great information and deals others find. If you have a Costco membership or CAA, some rental companies will offer discounts you can use as long as the reservation is in the same name as the card holder. Visa and MasterCard (and sometimes Amex) also offer discounts and coupons, but you must be able to pay for the rental with the card associated with the discount offer.
Our preference for car rental companies is Alamo, so we are often willing to pay a little more for them to help guarantee us a good experience. We like the fact that we get to select our own vehicle rather than be assigned one (we had a bad experience last Christmas with our assigned vehicle from Budget, so we are once bitten and twice shy). Whatever car rental company you ultimately choose, be sure to sign up for their free membership program as it usually allows you to skip the line at the airport or even (in Alamo's case, with their Insiders program) offer you an overall discount on the rental price.
