Dress in thin, wicking layers.
Start with a thin layer of synthetic material such as polypropylene, which wicks sweat from your body. Stay away from cotton because it holds the moisture and will keep you wet. An outer, breathable layer of nylon or Gore-Tex will help protect you against wind and precipitation, while still letting out heat and moisture to prevent overheating and chilling. If it's really cold out, you'll need a middle layer, such as polar fleece, for added insulation.
Protect your hands and feet.
As much as 30% of your body heat escapes through your hands and feet. On mild days, wear running gloves that wick moisture away. Mittens are a better choice on colder days because your fingers will share their body heat. You can also tuck disposable heat packets into your mittens. Add a wicking sock liner under a warm polar fleece or wool sock, but make sure you have enough room in your running shoes to accommodate these thicker socks.
Pay attention to temperature and wind chill.
If the wind is strong, it penetrates your clothes and removes the insulating layer of warm air around you. Your movement also creates wind chill because it increases air movement past your body. If the temperature dips below zero or the wind chill is below minus 20, hit the treadmill instead.
You're going to warm up once you get moving, so you should feel a little bit chilly when you start your run. If you're warm and comfortable when you first start, you're going to start sweating very early in your run. A good rule of thumb: Dress as if it's 20 degrees warmer outside than it really is.
Don't forget to cover your head.
More: Men's Winter Running Hats
Women's Winter Running Hats
Watch for frostbite.On really cold days, make sure you monitor your fingers, toes, ears, and nose. They may feel numb at first, but they should warm up a few minutes into your run. If you notice a patch of hard, pale, cold skin, you may have frostbite. Get out of the cold immediately and slowly warm the affected area. If numbness continues, seek emergency care.
Check with your MD.Cold air can trigger chest pain or asthma attacks in some people. Before braving the elements, talk to your doctor if you have any medical conditions or concerns about exercising outdoors.
Run into the wind.If you head out into the wind, it will be at your back at the end of your workout, when you're sweaty and could catch a chill.
More: How to Stay Hydrated Before, During, and After Runs
Don't stay in wet clothes.
If you get wet from rain, snow, or sweat in cold temperatures, you're at an increased risk for hypothermia, a lowering of your body temperature. If you're wet, change your clothes and get to warm shelter as quickly as possible. If you suspect hypothermia -- characterized by intense shivering, loss of coordination, slurred speech, and fatigue -- get emergency treatment immediately.