This may be common knowledge but it wasn't for me. The first time was a family trip to Scottsdale - four adults and two 3 year olds. Checked the big three and rented a Tahoe - and the cost for the week was almost $900 on all three. Ended up renting from National. A few days before the trip, decided to check again and snagged the same car, from the same rental company, for $350! Now we're headed to Florida - PBI. Same deal. Rented about 4-5 weeks ago and got a Ford Fusion from National for $380. Checked a few days ago and got the same car for $190 for the week. And looked yesterday and it was down to $160! Pretty easy - just reserve again and cancel the original one. Hmmm - maybe I should check again today!