To stay warm and dry the clothing you choose is extremely important. It can be the determining factor in having a good day or a bad day. There are several basic functions your clothing should provide.
First it should protect you from the elements. Wind, water and the terrain all need to be repelled. Let’s face it, if you are getting wet or the wind is blowing through then your jacket and ski pants are not doing their job. Your clothing also needs to provide you with enough insulation to keep warm. Layering your clothing will help trap your body heat better and will give you the ability to add or subtract layers as the temperature fluctuates.
Your clothing should be light weight and non-restrictive. You want to be comfortable in your clothing. Remember you will be in it for hours at a time.
Your ski jacket and pants are critical for keeping the elements out. The clothing must be able to breathe, this means it must stop water from getting in but it must also allow sweat to exit through it. Generally rip-stop nylon is used for the outer material. This will protect you from branches and other hazards like rocks and ice. Sandwiched in the middle is a layer of Gortex. Gortex is a must since it allows the clothing to breath; water vapor passes through from the inside to the outside only. This will dissipate your sweat and help keep you dry while keeping water from getting in.
Ski pants with a front bib are ideal since they add extra warmth to the chest and back plus eliminate the chance of any snow getting inside, unlike waist high pants. Much like the ski jacket, ski pants are made from the same materials. Their role is the same as the jacket, to protect you from the elements. Around the bottom of the leg are storm cuffs. The cuffs fit tightly around the ski boots and keep the snow from blowing into your ski pants. They are a must for keeping dry.
The layers of clothing worn under the jacket and pants should provide almost all of the warmth. Remember your jacket and pants are only for keeping the elements out. Generally an inner and outer layer of clothing will be sufficient unless it is exceptionally cold or windy. In extreme conditions a third layer might be necessary. Remember these layers have different purposes in themselves and consideration needs to be given when choosing the right clothing.
The outermost layer of inner clothing should provide most of the insulation. Generally a synthetic material like Polar Fleece is ideal for this layer. It has great insulating properties and it will allow moisture to pass through it rather then soaking it up like cotton. Once your clothing is wet it looses the ability to trap warm air inside itself. This layer should be fairly loose fitting but not overly large and should definitely be long sleeves.
The innermost layer of clothing should also fit the skin quite snug but not restrict movement or be uncomfortable. Avoid natural fibers like cotton for this layer as well since they absorb moisture which lowers their insulating ability. Polar Fleece is ideal once again. It will allow water vapor to be wicked away from the skin and be expelled into the air. If a third layer of clothing is needed a thin long sleeve skirt made from polyester should be sufficient.
Gloves are also critical. To truly stay warm and dry it is recommended to wear a glove that has a storm cuff which goes halfway up the fore arm. This will prevent any snow from getting into your glove and making you wet and cold. A glove with a removable inner liner is handy since the liner can be removed and replaced with a dry one part way through the day. If your fingers tend to get cold then you can opt for waterproof, breathable ski mitts instead of gloves. Mitts keep the fingers together thus generating more heat. There are also small hand warmers that are either battery powdered or a product like Hot Shots which use a chemical reaction to generate heat in a small pack.
Socks are often overlooked but play a very important role in staying warm while on the slopes. A trim fitting sock that doesn't bunch up is ideal. If the sock does bunch up it will likely cause painful sores on your feet. Ski boots fit quite snug. Polyester is best since it is less abrasive then cotton which can reduce the chance of blisters.
Your head and ears also have to be protected from the cold. A helmet can provide you with a small amount of warmth while skiing but they also serve another important function and that is protection your head from high speed and low speed impacts. An otherwise small crash can be very serious without a helmet. Packed snow is quite hard at 30 km/h. Helmets are also designed to let moisture escape, keeping your head dry, however, helmets on their own don't provide a lot of warmth. Of course, on a real cold day an additional layer of warmth might be needed on the head. Most ski shops carry helmet liners designed to fit under a helmet and provide warmth. A neck tube or full face mask are also great for those bitter cold days.
Keeping warm while outside in the winter is all about having good insulating layers that do not absorb water. When material gets wet it looses its ability to keep you warm. Using a material that doesn't absorb moisture is a must. Remember to always bring extra layers with you. It might be mild at home in the morning but when you get on the slopes the temperature may be much cooler. It is always better to have too much clothing then too little.
Layer your clothing so you add or take off layers as the temperature changes. Generally just having an extra long sleeve shirt will be enough. It is possible to stay warm and dry while outside all day. The tips above will allow you to do so.