What's the best way to keep liquids from freezing in cold weather?

Depending on your backcountry travels, before going out you'll want to heat up any water/drink/soup (until it's at least coffee or soup hot) and pour the water into an insulated 16 oz. stainless container. To help slow down freezing in winter add even more insulation to a Thermos or Nalgene bottle using a Nalgene bottle insulator.

Lastly keep the insulated container or nalgene bottle inside your backpack closest to your body (don't keep bottle on the outside of a backpack), or else keep the container inside your jacket for even more warmth.

For winter backcountry skiing or hiking day trips, it's also always a good idea to bring an ultra light stove, such as the Jetboil Minimo or a MSR PocketRocket stove w/ a TOAKS Titanium 600ml Pot. That way you can quickly boil snow into water in case of an emergency, such as running out of water, or if you need a hot drink to help stay warm.