Your trekking agency will provide equipment like sleeping bags, foam mattresses and tents. All you need to bring is your personal gear. We've seen porters make in through mountain passes in flip-flop sandals while carrying loads for two travelers. Nevertheless, we want you to enjoy your journey so use the lists (and your porters' muscles) below as guides.

Minimal Requirements 
Lightweight walking boots. "Walk them in" prior to arrival in Nepal to avoid blisters.
A pair of lightweight/heavyweight trousers are useful higher up in the mountains in the morning and at night.
1-2 pair of loose fitting long shorts/skirts.
2-4 cotton T-shirts.
1 lightweight long sleeved-shirt is particularly suitable for avoiding sun burn.
A sunhat and ensure it has wide brim to cover the face and neck.
2 pairs of thin and 2 pairs of thick woolen socks.
Underwear: normal quantity
Swimming suit
Water Bottle
Sunglasses and strap
Toiletries with large and small towels. Toilet paper can be bought in Kathmandu and some village in the mountains.
Small headlamp and/or flashlight/torch with spare batteries
Personal medical supplies - don't forget band-aids and twizzers
Army-knife and sewing kit
Sun-screen, sunblock, sun-tan lotion, zinc-oxide... get the picture?
This is essential in the Winter when skys are clearest. 
 
Winter Requirements 
Warm jacket. Fiberfill or down should be adequate. This is especially necessary during winter from December to February.
Sleeping bag to -15 C or sleep sheet (if renting or agency supplied)
Woolen shirts and thick sweaters. During winter months, December through February, These items are essential. Thick sweaters can be purchased in Kathmandu.
Windproof/Waterproof trousers. Necessity on all treks going above 3,000 meters.
Thermal underwear. These are excellent to sleep in at night. In the winter months thermal underwear are quite invaluable.
A woolen hat to wear in the morning and at night. During winter it is an essential item.
A pair of gloves. Leather with lining and woolen are best.
Snow Glasses and strap
Snow gaiters can be essential 
 
Some nice add-ons
Camera & Film
A pair of slip-on shoes or sandals. To wear in the camp, in bathroom and toilet tent or when the boots are wet.
A rain-proof jacket with hood or a poncho. Get the one that is guaranteed waterproof.
A sweat-suit. Useful for wearing in camp and in the tent.
Duffel bag or kit bag to carry gear while trekking.
Daypack. This is a small backpack to carry personal requirement for the day e.g., to toilet items, camera, film, towel, soap, a book etc.
Spare boot laces.
2-4 large plastic bags to separate clean clothes from dirty ones. 6-10 smaller plastic bags to dispose garbage.
Wallet and/or money belt with compartment for coins.
Spare flashlight bulbs, candles and lighter to burn toilet paper.
An umbrella is quite useful as a walking stick, a sunshade and for rain.
Reading materials, game items, music, note book, rubber band, pen and pencil envelopes, a diary, a calendar, a pocket knife, binoculars (optional), A small pillow or headrest (optional) Thermarest (optional) - an inflatable sleeping mat, trekking map, adequate quantities of passport photographs.
Duct-tape, superglue and small mirror can be handy
Travel locks and chain to secure luggage and lodge door
Hot-water bottle - unless your mate's comin' along 
 
Unnecessary Items - reminder...
Cell-phone, Pager, Lap-top computer, PDA, etc
Radio, TV, magazines, newspapers, etc

Please Note: North Face and Pategonia type companies own the market in the USA, but many of the above mentioned items can be purchased/rented in Kathmandu and Pokhara. These high-tech companies and there products can make for a dreamy trekking experience when you learn what all thoughs pockets and fancy fabrics are really designed for.
Beware: Virtually all the brand name items in Nepal are Korean knock-offs. Shoes and sox are the essential items to bring from outside the country, if your in the Bigfoot category.