One thing people don't realize is how much water you breathe out at altitude as the air is much drier. You can easily lose a quart of water in your sleep in cold weather at altitude and then start your hike already dehydrated. I would rather carry it in me than on my back so I drink as much as I can, over a half gallon, before I hit the trail. You should be peeing pretty often, and clear(unless you're taking vitamins that change the color). Hydration is probably the easiest way to combat the effect of altitude. A headache is the most common sign that you're not drinking enough.
