The cost of your flight can be the most expensive, not to mention confusing and stressful, part of planning any trip. You know that the purchase price is an ever-changing and evolving beast. One day it's surprisingly cheap, the next it's prohibitively expensive. There are all sorts of sites and apps that are designed to help you but where do you start? How do you prevent yourself from overpaying? We've distilled the best expert advice into six cardinal rules. Follow these and you'll be sure to have plenty of cash left in your pocket to spend once you get to your destination.