Water

I am a complete fascist bully-boy when it comes to water! I have seen folks take 1 pint (600mls) or less on a day hike, strenuously arguing they don't need to drink much. In the same vein, I know people who equate caffeinated (ie. diuretic) drinks like tea and coffee as suitable substitutes for water.
Why am I so militant? First, when resting the average person loses 2-3 liters of water by transpiration alone. If you are sitting calmly in the shade at mild temperatures, unless you have 2 liters of free water coming into your body, you will incur a net loss. Add walking, running, carrying loads into the mix and you will lose an awful lot more in sweat. Second, you would probably survive 3 days without water until your body starts some serious metabolic shutdown. I saw on one of the ultralight hiking sites about a gentleman who hiked the Pacific Crest Trail with a load of 9 pounds (4kg) including clothing, fuel, food, and water. All the more power to him, but he can't have been carrying much water (1 liter = 1 kg) so I assume his resupply points were pretty close together. Third, dehydration comes quickly and it kills. Soldiers in the army I served in have died on training exercises, and I personally have had to arrange medivacs for soldiers who have gone down. Importantly, even minor dehydration causes lapses in awareness and judgement. This is not a good thing. Finally, I am biased because I have worked an awful lot in very dry areas where there is basically no surface water. There were no convenient streams to resupply from. Obtaining enough water is a large task in itself.
In the army in marching order (pack, webbing) I carried 8 liters. I would resupply whenever I could. The onset and effects of dehydration are well known.

Dehydration levels and effects on the body. Loss in liters is based on a body weight of 70kg. Number of days is based on a loss of 2 liters per day
Fluid loss (%) Fluid loss (liters) Symptoms Days (maximum)
1-5%
0.7-3.5
Thirst, discomfort, appetite loss, flushed skin, irritability, sleeplessness, nausea
1-2
6-10%
4.2-7
Dizziness, headache, breathing difficulty, no saliva, slurred speech, unable to walk
2-4
11-20%
7.7-14
Delirium, swollen tongue, swallow difficulties, vision blurring, numb and shrivelled skin
4-7
Serious trouble!
Body and brain start to shut down
Pray for rain
Let's look first at a 'normal' ie non-emergency approach to water management which I consider elsewhere.

Carrying Water

store.nalgene.comAside from the obvious need to have a container of some sort, as water is heavy (1 liter=1kg) there are weight, loading and accessibility issues to consider.

I mentioned previously under Packs and Load Bearing that some sort of belt with canteens on the waist is a good place to keep water. Water is within reach, and low on the body. In standard army webbing, typically 2 1 liter bottles would be carried. I carry 4 liters. Kidney-shaped army issue bottles are a bit bulky but OK. 1 liter Nalgene or similar bottles fit nicely into a rectangular pouch. I mentioned before to avoid the loose netting pouches on some packs, unless you like losing water bottles. Why be fussy about the size of the bottles? Carrying 2 liters on your waist ensures you are getting a minimum for the day. If you haven't had to resupply from your pack during the day's hike - you are in water deficit.http://www.camelbak.com/en/International/Military-Tactical/Packs/ThermoBak-3L.aspx
Camelbaks or a similar generic are a good way of carrying and ensuring you drink water. The weight is close to your body and the drinking tube is readily accessible. I use a generic brand on the rare occasion I'm not carrying some sort of load bearing equipment, for example when orienteering. When writing this page I noticed that Camelbak has gone a wee bit to the over-embellishing dark side. All you need really is a basic harness and bag to hold the bladder.
Keep it light and simple.

Filtration and treatment

I was kind of lucky during my early bush days in the 'old country'. Water was generally available, and I thought little of drinking it. In other places however, it may be necessary to treat collected water. Physical filtration byHiker Pro cloth removes little except mud. I was issued a Millibank filter in the army - needless to say it lived my entire career in my third trunk. You may as well use a sock.
However, in areas with high livestock densities, poor hygiene - basically a lot of the world - bacterial-borne diseases, Giardia, Cholera and so on are real risks. Boiling water for about 20 minutes is always advisable, but seldom practical. Filters such as those made by Katadyne and MSR are pretty good. I have a Katadyne filter and it's great - but honestly I rarely carry or use it.This is simply because I'm either not relying on natural sources, or I'm comfortable that natural sources are OK. Purification tablets are a wise item to carry. They are light,Water purification tablets easy to throw in your water bottles, but unfortunately taste like crap. Hey - it's better than being violently ill, with eggy-burps and farts, or worse.

Conservation and rationing

Given we have to carry our water, it makes sense not to waste it. This brings us to conservation and rationing - which are not the same thing.

Conservation

Water conservation is about maximising the effectiveness of water use without compromising your need to actually drink the stuff. Some of the key points are obvious, others less so.
Use water only for drinking. Your socks can afford to go unwashed for a while. You can go without a shave for a while. Even though it may be a bit warm, water should not be used pour over your body in some dance scene from Flashdance. While field hygiene is important, the dishes can make do with being scraped and dry-rubbed clean rather than soaped up.
When you have it, drink it. At water resupply points, to use an army term - Bomb-up. Get all water-related ablutions out of the way, and pound a liter or two. However... this influx of water will tend to flush through you. Ease up before you set off again to get your body back into 'no wee-wee' mode.
Drink it when it is most efficiently taken up by your body. Metabolic loss is relatively unavoidable, except by controlling workload (see below). Water is better absorbed into your body when you are at rest, and not just going to sweat it out. Yes, water is a necessary coolant. But you will make better use of it by drinking it in the cool of the mornings and evenings. That being said, small mouthfuls (sips) during high workload will be necessary. Resist the urge to pound down large volumes though - it will just go through your body.
Avoid diuretics and be judicious with salt. Hard to believe, but coffee and tea are not substitutes for water. They increase water loss by urination. Similarly, alcohol should be avoided if water is limiting. Except for single malt scotch whisky, of course. There are limits to deprivation, after all. Salt tablets have been recommended for decades to reduce cramping due to salts getting sweated out. Increases in salts in the body increases the osmotic gradient between the blood and the cells, so water in the bloodstream will lose water to the cells. I have never - even in extreme sweatiness - had any need for salt tablets. My diet is usually sufficient without added salt. I'm in agreement with Richard Graves on this one. If the Kalahari bushmen think it's nonsense, well common sense and a basic understanding of physical chemistry indicate if you are water-deplete, salt is a bad idea. An additional word on 'isotonic' drinks. You may as well just throw sugar and salt into some water and drink it while burning wads of money. Water by itself is the best rehydrator. D'Oh! There goes my potential sponsorship by you-know-who.
Ration sweat, not water. Cooling (sweating) is the greatest source of water loss.Water requirement graph Minimizing workload during the heat of the day and shifting it to cooler times will increase water conservation dramatically. Let's say you want to cover 20km in a day. Why not have an early start, break in the heat of the day, and restart as it begins to cool? I hate to use the old 'war stories' but on one army exercise we were operating in open country in about 38-40 degree heat, full packs, kevlar helmets and body armour. At night it didn't cool much, and we were digging in. We required 3 water resupplies per day. That is 3 lots of 8 ie. 24 liters consumed over the day. We still got roaring dehydration headaches. My civilian friends don't believe it (How can you possibly drink that much in a day!) so here's the graph of the US Army's study. Ration sweat!

Rationing

Rationing is a more serious situation in which water resupply is not assured, and the prospect of running out before resupply is real. This approaches a Survival situation. In a survival situation aim to make 1 liter of water last about 4 days at 1/4 liter per day for 3 days. The last 1/4 liter is then made to last 3 days at about 80 ml per day. This 80 ml should be taken over 2 doses. Say, midday and night. This is a modest mouthful.
Needless to say, this is an extreme situation and even a glancing reference to the dehydration table above will illustrate how much the limits of water deficit are being pushed. You will be only be slowing the entry into the second and third stages of dehydration, and even then not by much.