While trekking lower down in the valleys it is hot and you sweat a lot, so it is important to replace the fluids you lose. At altitude the problem is worse, you are still sweating and the air is dry meaning you must breathe a lot harder. With every breath you breath out water vapour. It is very important then to drink a lot. Dehydration makes you feel tired and lethargic and can give you a headache. Therefore always keep hydrated.
The basic rules are; drink as much and as often as you like, (that does not include alcohol!) even if it seems like a lot. This can include soups and lots of tea, but even with a lot of liquid food, you should drink a lot of water too. Many people find that with dinner they often drink more than a litre of water, catching up on what they should have drunk during the day. A great guideline is the expression: A happy mountaineer always pees clear!
The easiest way to check that you are not becoming dehydrated is to look at your urine; if it is very yellow or orange you should drink more, but if it is almost clear then you have been looking after yourself well. Using this as a guideline some trekkers find that, although they don't feel thirsty, their urine is definitely yellow. This means drink more, even if you have to consciously think about it.