What's the best design for bottle Rockets to yield the longest flight time?

I'm working on a bottle rocket for physics, made out of plastic bottles, small or large. We get points based on the flight time and also e.c. if the flight time is more than seven seconds. That is, the flight time includes the duration from launch until it hits the ground with the parachute (plastic bag). It will be propelled by pressurized water. I'm wondering it there's any tricks involving the designs that will yield the longest flight time?