Rental car is usually the first thing I book for a trip, often as far as 8 months in advance, because you can cancel and re-book without penalty as much as you like (up till 48 hours in advance).
I will typically book as soon as my dates are set, and re-book, sometimes 3 or 4 times, when a better deal comes along. I usually wind up paying less than $150 for a week-long Economy or Compact-class rental, and sometimes I get coupons for class upgrades to Mid-Size class.
I always browse MouseSavers.com for discount codes. But the best deals I get are almost always with the codes and coupons I get as part of my
Sams Club membership (Costco and BJs have codes like these, too).
It's also a good idea to join the frequent renters clubs at several of the companies. Most of them are free (Dollar has Fastlane,
Alamo has Quick silver, National has Emerald Club, Thrifty has Blue Chip, etc). These memberships usually won't get you better rates, but they will allow you to fill out a master rental agreement (contract) ahead of time, which the company keeps on file, and then you don't have to fill one out when you pick up a car, speeding things up a lot. They will also prevent you from getting the hard-sell insurance pitch, and most times you go to a special (SHORTER!) line when you are a club member.
If the current good deals expire in Feb and March, new good deals will come out just before the old ones expire. But you should shop around - sometimes a great deal at DOllar will end, and a good deal at Alamo or Thrifty will start. This happens at various times thoughout the year, so you will need to check back about once a week to see if your rate drops.
Don't be afraid to cancel a ressie and make a completely new one with a different company. This is how you get better rates! I have often started with a $200 rate and whittled it down to $100.
Just keep in mind - when you check rates, the "base rate" does not includes the taxes and other fees that must be added on to get your final total. Most companies will give you the estimated total on the last screen before you hit the "Make reservation" button, and you can compare the final totals.