I have had good luck with National and
Alamo in the past, mostly due to their frequent renter programs. Sign up for both a few months in advance (free links on Mousesavers.com). They will send you coupons via email. When you rent an intermediate with them you have a good chance of getting a larger car for the same price because you pick from any vehicle on the aisle.
I tend to use the orbitz and expedia just to get an idea of what prices are.
When I see one I am comfortable with, I book it. I then continue to watch the prices every week or so. When they drop, rebook. When you get a coupon, try rebooking. If the price is not lower, call the agency and tell them you just got a coupon and would like it applied to an existing reservation.
Also highly recommend watching the car rental board for your month to see what teh deals are.
In the past, I have found the best deals either way in advance or last minute. Our first trip I got a mini van with National for $325 for 10 days in Jan. - booked 6 months in advance and never saw a better deal. Next trip I rented an Int. with Alamo for $201 for 10 days (also Jan.) (booked the day before departure) and we found a minivan in the int. aisle.