With regards to altitude sickness, drink lots of water. If Yellowstone is at a higher elevation than what you are used to, water will do the trick. Your body is trying to add oxygen to your bloodstream and if you don't have enough fluids in you, it is hard to make the extra blood to carry the extra oxygen. Day hikes up the mountains generally won't cause altitude sickness if the base elevation of the mountain is what you are used to.Some swear that the enzyme CoQ-10 works great, too, as it helps your cells oxygenate better. Start taking the CoQ-10 about two weeks before you leave. You can find it in the vitamin aisle of your favorite place to buy vitamins.
Enjoy your trip!!