Travelling takes a toll on your metabolism because of the long hours of sitting that you do. You can battle this by stopping every few hours and walking about for 10-15 minutes or just doing a few sets of 20 squat jumps.
This is a bit more challenging when you fly, as they generally frown upon using the isle for sprints during the flight. But you can try adding a few unnecessary restroom trips to get some more movement. If that bothers your conscience then take a 40 ounce bottle of water and try to finish it before the flight takes off. You need to drink more when you fly anyways, as it has a tendency to dehydrate you. In addition, utilize your layovers wisely. Rather than sitting, take a tour of the airport, and maybe even do a few body weight squats to mix thing ups.