Sign up for email spam from the car rental companies, they often have last minute special codes. This is good even if you don't want to rent as you can also forward to family flying into town that wants to rent a car.
Also, don't just walk up to the counter, check the website specials first (ex. at Alamo.com, click on the "Hot Deals" tab at the top of the page for the discount codes).
Also, if you are a Costco member, go to the Costco.com travel section and see what the codes are for various car rental companies, that is often the cheapest for me.
Also, your employer may have a corporate ID which you might be able to use to leisure travel as well as work that you can use with their preferred rental company, sometimes that is the cheapest way to go, esp in high tourist season.