Here are a few notes that I made after suffering from mild altitude sickness when we were in Lhasa, Tibet. Please talk to your doctor before going on a trip that involves high altitude. There are also many websites with good information, so that you are well informed.
Nausea and headache are two common symptoms.
Vomiting is not good.
Stay a minimum of 3 days at high altitude to acclimatize before going further up (if possible).
Drink LOTS of water (2 litres a day).
Breathe deeply and regularly. Concentrate on your breathing!
Maintain low exertion.
Use oxygen before sleeping to help you sleep.