In the course of preparing for the Gokyo Ri Trek, I've been asked for my opinion on staying hydrated. For me this topic breaks down into 3 aspects: how much, how to sterilize, and what sort of bottle.
So first of all, staying well hydrated in the mountains is incredibly important. If possible you should keep an eye on the colour of your piss as this is a great indicator of how well hydrated you are. Here's a link to a handy chart. The darker the colour, the more water you need. You can figure on drinking a minimum of 4 litres of water/liquids per day. Teas and soups can count toward this goal. I personally try to drink ~6 litres per day which I manage by drinking a litre of black tea with each meal and then at least 1 litre of water during the morning hike and the same during the afternoon hike. If you are taking Diamox to stave off altitude sickness then this becomes even more important as it affects your kidneys. In this scenario then you should definitely be aiming to drink at least 6 litres per day.
Next we move on to sterilisation. There are a number of options here and, of course, this is just my opinion. I know that some people use UV with products like the steri-pen and some of my friends have used these quite successfully for extended periods in places like India and Nepal but I discussed this option with my brother who is a chemist and he has steered me away suggesting instead that I consider a Life Straw. I have used neither so cannot speak from experience but the fact that the Life Straw is distributed throughout Africa and is endorsed by the United Nations makes me think that it might be worth consideration.
Next we move on to chemically treating your water and here the decision is between using iodine or chlorine. To me, Iodine has several disadvantages in that it loses its effectiveness when exposed to sunlight, must be used with warm water if Giardia is a concern (which is true for the Himalaya), and some people (like myself) have a sensitivity/allergic reaction to iodine. So I personally use cholrine tablets. The effectiveness of these is also affected by water temperature so if your water is cold then it will take longer for them to work. They also affect the taste of your water but whether you use iodine or chlorine, you can add neutralising tablets or some flavouring to the water. I don't bother as the chlorine taste does not bother me and in fact reassures me that the water is safe.
Finally, what sort of vessel should you use? I've bought one of the CamelBaks because on the face of it they seem like a good idea in that the water stays in your backpack with just a tube coming out for convenient sipping. That said, I haven't actually used mine yet because I haven't figured out how I would keep it clean. Also, when in Nepal last year I saw some poor girl slip on the rocks and when she fell her drinking tube fell into a pile of yak shit so I would suggest that if you use this option then make sure you pack an extra drinking tube.
I've tried using water bottles and they're fine but I would caution you to ensure that your bottle is BPA free. I don't use them because they are bulky and I strive to keep the volume and weight of my daypack to a minimum. Instead, I use a collapsible water bottle (I suppose it's really more of a pouch or bag) so that as I drink the water the bottle becomes less bulky.
I've also been asked about buying bottled water and while this is certainly available in the himalaya (at least in the places that I have been), it's not really the done thing to buy water because it needs to be transported there and adds to the trash/recycling problem unless you're planning to hump it in and out yourself.
If you have other questions about trekking and/or preparation for trekking, please put them in the comments and I'll try to address them. In the meantime, please don't forget to donate to The National Museum of Computing to support my trek and the important work done by the museum to preserve our computing heritage.