We went through this for last January and I just did it again for this coming January, for Florida.
I play around with discount codes, contract IDs and coupons. You can find them by googling what you want, and then plug them into online reservations sites.
Different codes do different things. Some may lower the price of luxury vehicles, minivans or SUV and some my lower the price of economy vehicles.
I found one this year that made a Cadillac DTS less expensive than a full-size. I know it sounds high, but after adding all the fees, $338 for a week is not that high any more. The pre-discounted rate for the Cadillac was close to $800. Sometimes value makes a higher price a better price.
Then I had to redo it, but for two weeks. When I did that a Standard SUV came out best, at $572.
Last year without codes the best National Emerald Club rate was over $1200 and I got it below $600 with a code I found on Flyertalk. For some reason I couldn't find that forum this year.
The codes don't seem to be as good this year and a lot of them, especially the coupons, expire at the end of the year.