Change your watch to the local time as soon as you remember. If you're crossing time zones, the sooner you get on track with the local time, the better for sleep.
Try to avoid napping on a daytime flight, and you may sleep better that night. If you're flying overnight, try to keep your sleep patterns on track and get as much rest as you can. Using an eye mask and earplugs can help.
Get up and walk through the aisles every once in a while. While you're sitting, take frequent breaks to stretch your arms, torso, and legs, and make circles with your feet. Stretching can help prevent cricks and cramps that could keep you up later.