Usually for me, fan at 2 or 3, thermostat in the middle. If it starts getting cold, set it lower. This is more fuel efficient than leaving the fan on 1 then turning the thermostat all the way to the "cold" setting. Setting the fan at 4 doesn't help... the fan will be working hard enough to heat the motor (notice how ACs never seem cold when you set the fan to full blast?) and moving air also causes more friction, less cooling.

I also toggle the A/C button on and off in traffic. I turn it on again when I'm back at cruising speed. I almost never use it at night.