Every year many hunters and outdoorsman and women come out west from the midwest and east coast to hunt the prized mulies and elk. One topic that comes up often is altitude sickness. My advice for flatlanders is to get into the best possible shape. Start months before your hunt, usually really ramping up my cardio around March or April.
I run 5-10 miles 3 times a week, and also go for walks carrying my pack with about 50lbs to simulate what could be on my back. Another useful tip is to drink A LOT of water before, during and after your hunt. If you're not used to high elevations try to get to your hunt area a few days early and camp out at lower elevation to get accustomed to the change. Remember to drink water. After a day or two of lower elevation camping make your way to the higher elevation but take your time and drink a lot of water as you go. Moral of the story is get in shape, drink plenty of water and take your time to get accustomed to the elevation and it should make for a way better hunt.