For in-traffic use, try installing a second brake-light switch so it comes on when you barely touch the brake pedal. It trips a relay and switches the A/C on. This enables you to regulate your speed in traffic. Keep the a/c fan on and the air stays mostly cool. But instead of turning inertia into powdered brake-linings, you'll start turning it into cool air.

The compressor takes the lease load when first turned on because there is equal pressure on both sides of the pump. Turning it on just after it stopped with full pressure uses the most energy and in 110AC powered cooling systems the motor may actually fail to start and kick in the thermal current overload switch and that really wastes power. In a car with a clutch it presents the worse shock load to the clutch and belt when you kick it on and off a lot.

Best to keep the fan on low the temp to the coldest setting and shut it off when you need to accelerate or rev the engine more. Turn it on when slowing down or going down hill and then kick up the fan as well to create more braking action and make the compressor work harder when you want to slow down. Most important is to recirculate the air in the vehicle so that it dries out the air and can cool you more as well as to keep cooling the already cool dry air in the vehicle instead of constantly taking in the HOT outside air. Keep on eye on engine temps as well because the AC can cause more engine heating when stuck in traffic. Also keep some distance from the vehicle in front of you to allow more cool air to reach the radiator and condensor or else you get the heat of the one in front of you and reduce the cooling efficiency. I barely notice the AC in my Scion when used in this way and the impact is maybe a mile per gallon which is a small price to pay for a lot of comfort and not sweating up the seats.

Remember to let a little fresh air in once in a while if using recirculate or the vehicle will get a little stuffy - low on O2 high on CO2 !