Bring a LOT of Water Treatment Tablets
Since drinking water is so important on the trail, make sure you bring plenty of water purification tablets (we used these Aqua Iodine and Taste-Neutralizer Tablets ($12)). If you drink five liters of water every day, you're on the trail for 15 days, and you need two tablets per liter, that's 150 tablets. And that's not counting the water you need to treat for brushing your teeth and washing your hands and face.
All tea houses sell bottled water, and we definitely bought a lot on our trek, but you always need to be careful that it's sealed. Our Sherpas warned us of some places filling bottles from the tap and repackaging them, which will make you sick. Treating all water is your safest bet!