Get up and move around on planes, trains, and when driving at least every 90 minutes. On planes, plan ahead since bad weather might keep you in your seat past the time you should be up and moving. This is an important piece of advice that is often overlooked, but pay hindrance.
Prolonged sitting can allow the blood to form clots in the legs, and then these clots can be released when movement returns. A free clot is called an embolism and these emboli can lodge in distant areas of the body causing devastating problems like strokes and pulmonary embolism.
I have two personal bad experiences with people who I knew who did not pay attention to this rule:
1) I was on an overseas airplane trip with a person who did not get up from her seat for many hours. When she finally got up upon arriving, while she was walking off the plane, a clot that had developed in her leg released into her blood stream and she had a disabling stroke.
2) The second case was even more tragic. A patient of mine was riding in a car for three and a half hours without stopping and when she arrived at her destination, a clot released from her legs into her circulation, went to her lungs, and she died of a pulmonary embolism.
Remember, get up from sitting at least once every hour-and-a-half and move around for a few minutes. It can save your life.