100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
88) Cycle the A/C if you have to use it
If you have to use the air conditioner, set the air flow to recirculate and manually turn the A/C on and off as needed.
For greater efficiency, switch it on when under light engine loads or deceleration fuel cut off and off when under moderate/heavy loads. (Note: some newer vehicles do this automatically.)
At least on my car, having used the AC, the fresh air apparently gets drawn in over the chilled cooler, and remains cold (colder than ambient) for a while.