<human>:  What is the most cost-effective way to fly round-trip from Denver to Dulles?
<bot>: The most cost-effective way to fly round-trip from Denver to Dulles is to book in advance and compare prices from different airlines to get the best deal. It may also be beneficial to fly during off-peak hours, as flight costs tend to be lower at these times. Additionally, flying mid-week (Tuesday, Wednesday, or Thursday) may result in cheaper fares than on weekends.