I have used CoQ-10 (200 mg 2x day) and Ferrum Phos tissue salts (follow directions on the bottle) in combination to help combat altitude sickness. Start taking them a week before you go and keep taking the entire time you travel - they help with energy and getting more oxygen around the body.
Adaptogens like reishi or cordyceps help also.
I get terribly short of breath and bad headaches at as low as 4,000 feet but the CoQ-10/Ferrum Phos combo allowed me to go to a conference in Colorado and be perky and energetic, I was even able to take a walking tour around the town. Haven't tried it yet above 8,000', but I bet it would still help.