You will get lots of answers and opinions here
Everyone has an opinion/experience with different car companies.
Just my personal opinion
I usually use Travelocity, Expedia etc. as guides to what the going rates are. I also frequent the boards looking for codes. I'm signed up with several of the rental companies to receive email's from them advertising their specials. I tend to stick with Dollar, I have the best luck with rates with them. and for the most part, I'm happy with their service, I use them when I travel other places too. I go directly to their site and plug in dates, times etc., and any codes I've found. Even after I make a reservation, I check back frequently up til my trip, using different codes or heeding advice I've seen on the boards about best deals and make a new reservation at perhaps a lower rate.
The best advice is to do your homework, be diligent and listen to others here on the board, you can find a wealth of info and also save yourself some $$$. Case in point, I've had a ressie with Dollar since January for an upcoming trip, and by keeping an eye on it, I've lowered my weekly rate 3 times saving about $100 over the coarse of my 16 day rental need.
Hope this helps and I'm sure others will give some advice too