Hydration really helps with the altitude sickness, I also have a hard time the first few days, so I slower my pace and listen to my body...then my body adapts. I do find that I have an easier time with the altitude when I'm in better shape, but as the other gals mentioned, this isn't true for everyone. I still dread the initial high altitude days.

Also forgot headlamp, sunscreen, and chamois creme on my list. Chamois cream is a personal choice, but it sure makes life a little easier for me.