This is taken directly from this month's Gardening Life Magazine:

Option 1-Keeping Container Outdoors:

Wood, fibreglass and plastic containers weather best; clay may crack.  To insulate roots, sink containers into the ground or wrap the container in layers of heavy duty plastic or bubble wrap and then burlap to endure the cold. Place stakes in the pot and wrap burlap around the plant to protect from wind and heavy snow.

Option 2-Move into a garage or unheated shed:

Water the plant until frost and then throw snow on soil surface to supply gradual moisture. Elevate the pot on bricks or chocks.