This packing list should work as long as you are travelling with a trekking tour operator. If you are organizing your own trek, please refer to the packing list when you are organizing the trek.
Clothes:
3 changes of clothing (preferrably layers)
Inner Garments, long johns
Wind and waterproof Jacket
Sweater/fleece jacket
Trekking shoes
Socks (3-4 pairs cotton and a pair or so woolen)
Floaters/sandals/slippers
Balaclava/woolen cap
Gloves – good quality is important
Hat/sun cap
Gear:
Ruck sack to pack everything into
Waist pouch for valuables
Water bottle
Sun glasses (very dark, avoid blue), spares for normal glasses
Walking Sticks (optional)
Head lamp/torch, spare batteries
Camera/binoculars, etc (optional)
Personal medications/products:
Sunblock
Cold cream/vaseline
Tooth paste, tooth brush
Soap, comb, etc
Personal medications – minor OTC necessities as well as any prescription medicines (ashtma inhalers, insulin, etc)
Other Stuff:
4 Passport size photos and a valid identity card
Clothing for high altitude treks should be planned to provide insulation. The air is invariably colder.
How warm depends on how high you are going, how cold the region is and the time of the year. The guidelines here should work for all three as long as you scale them up/down as per your requirements.
Plan your clothing in layers. Layers trap air between them and provide more insulation than one thick layer. They are also easier to pack.
Full sleeved T-shirts work very well, and three full sleeved T-shirts are warmer than most sweaters. The same logic applies to trousers, socks.....
Many people like to walk in shorts. While not life threatening, I've seen my share of bright pink thighs at the end of one day at high altitude. The more skin you leave open, the faster you are going to dehydrate. Its also invitation to scrapes easily prevented by clothing. If you can deal with it, and you can't bear not wearing shorts..... wear them in warm weather and green areas and preferably on rest days, so you can change when it gets too much. Go for sunblock, drink plenty of water, and if you get a headache, don't come running here.
That goes for the sleeve less clothing too - actually, shoulders will burn faster than thighs, and then you are going to not like carrying your sack. :D
Most mountain cultures in India are conservative, so if you are in or near civilization, wear full clothing in any case if you want the locals to be comfortable around you.
Another practice I have seen is removing warm clothing after starting walking. Again, not a good idea. Stopping to remove clothing already starts cooling your body down and leaves you vulnerable to the cold. Strange as it sounds, unless you are feeling very hot, the layers also protect you from the heat around you.
What they basically do, is keep your body temperature uniform, which is nice for your health. Think of every picture of the mountain people from the Himalaya you have seen - their woolen coats are like a uniform - day and night. These guys live incredibly hardy lives under the sky round the year, but they will not even take their coats off indoors. They wake up and put their warm clothes on and take them off before they go to bed. They also very rarely fall ill because of either heat or cold.
A comparable example from city life would be making trips in and out of an air conditioned area in sweltering heat. Remember the head ache?
About the quantity of clothes to take..... what I usually do is take about 1 thin and one thick T-shirt for every 3 days of the trek. Uh.... yes I don't change everyday, but if you're picky, carry a few more of the thin ones - they will not take much space in your sack. Change the inner ones and use as many thicker ones on top as you need - they will go for days without making you feel unclean in the slightest way (or perhaps you get used to it).
The only thing worse than the fear of death is the fear of a wasted life.
-- Bill Hybels