I would start now. I already have a reservation for my March 6 - 13 trip. As long as you don't pre-pay you can cancel one reservation when a better one comes along. Last year my first reservation(made 6 months out) was the one I ended up using. A good starting place is http://www.carrentalsavers.com/. Also check out the discounts on Mousesavers.com. It is really early for March codes, but it never hurts to start looking.
If you are a costco member, you can book through their site for a decent discount. I have a car booked for March 11-23 but it usually goes down as we get closer to the date. I just cancel and rebook each time it changes.