No one has mentioned vehicle speed.
If you live in a city environment (as I do) and you are only going a mile or two, it pays to roll the windows down because, aerodynamically, it won't matter if you are never going to go any faster than 20-30 MPH. That's assuming you can stand the heat and humidity, depending upon where you live. Better yet, consider owning and driving a convertible for summer months.
If you must have the A/C on, and there are times (believe me, I know) when you can't manage to do without it, short of being sweat-soaked to the skin, put the A/C on recirculate, shut it off while accelerating but turn it immediately back on while decelerating or coasting, or while at a stop light.
If you are going to be driving at highway speeds, shut all the windows and turn it on max cold + recirculate. If you get stuck in traffic, the same strategy as stated above applies. But after you have been driving a while and once the interior is already cold, it is best to keep it cold, rather than start all over again.
