It is common for people to experience mild symptoms of Mountain Sickness when they come to Tibet at fist. You need one to two days to get used to the plateau condition. As your body gets used to the altitude, they symptoms will go away. You can try to do some exercise before you come to Tibet. Normally people will take some medicine like Hongjingtian 10 days in advance to avoid Mountain Sickness. And another common recommended medicine would be Diamox which many foreign travelers prescribe it before coming to Tibet.