Carry a comfortable sweater to slip on or off. Cabin temperatures may change even on a short flight. On a longer flight, climatic conditions may vary enormously. If the flight you are on allows smoking, avoid it. Smokers are more adversely affected by high altitudes than non-smokers.
Tobacco smoke hinders the body's ability to absorb oxygen.
Make a point of drinking plenty of liquids before and during a long flight. At high altitudes, the aircraft cabin is extremely dry. The best drinks are water and fruit juice. Avoid alcohol, it dehydrates the body. Coffee and tea also tend to dehydrate, so drink them sparingly.