I myself will be out in CO from the 11th to the 23rd chasing elk with my bow as well. This is my 7th season and I myself have NEVER had altitude sickness. After speking to my mom who is a retired doc, she said that some people get altitude/motion sickness and some people dont. I agree with the potass and iron and being in VERY good cardio shape. I go to the HIGH county and start most my hunts at 8,500'-9,000' and frequently end up at 10,500'-11,500'. You should also try shooting your bow after your heart rate is above 130BPM. I have seen fist hand a lot of people including myself not be prepared for the shot after running up and hill and not being able to close the deal cause they where not used to shooting while breathing hard.

EH