Idling for just 30 seconds burns more fuel than turning off the car and restarting it. Turn the ignition off at long stoplights and other lengthy hold-ups. And rather than using the drive-through, just park and go in. When I do use the drive up I will turn off my car if the wait is long (especially when using the prescription pick up at Walgreens)
I know this is hard in the summer which is why going inside is the best option. Besides that I read that you can burn 8 pound a year by parking and walking in instead of using the drive thru.