Okay, actually I overwintered bonsai and cps in their growing pots. Dig a hole at least twice as deep and as large around as the pot. Layer mulch (leaves, straw, what ever, newspapers in a pinch) on the bottom about 1/4 of the way. Place the pot in the hole use the rest of the mulch to fill in around and over the pot but leave the plant itself exposed to the elements. Then place peat moss over the top to keep the mulch in place. (for bonsai I would substitute soil for peat moss). if you have small pots they can all go in the same hole, just put some mulch between them.
This prevents a freeze of the media within the pots. The mulch acts as insulation bu the plant goes into a natural dormancy. I've had 2 ft of snow cover my plants, then ice form over the top without a loss.