Acclimatizing by sleeping at progressively higher altitudes is the best way. Your body produces more red blood cells while sleeping at high altitude. I live at 4000 feet. I spend a night at 8000, followed by 10000, before working hard at 12000 plus.
If you live at a lower altitude, you will need to start lower.
If you start to have symptoms, make sure to get to lower elevations as soon as you can.