I'm a double carpool veteran and I'll say they've saved my bacon: even with two kids at two different schools, I only drive 2 weeks out of every month, which has saved both gas and sanity. In both cases:
1.) One family drives to and from school, for a whole week.
2.) Be on time, both as a passenger and a driver. It's a good opportunity to teach a lesson in courtesy to your child. It is much, much harder than it sounds. We fail, frequently and repeatedly, but this is our goal.
3.) For middle school girls, for the morning pick up - I recommend a 5 minute warning telephone call. "They''re on their way to get you" seems to work better than repeated parental nagging at coaxing a girl out of the bathroom where she is fixing her hair.