First things first....let's get one thing out of the way. Rabbits can tolerate cold better than they can heat.
Rabbits can handle cold but no drafts. Provided they are kept out of drafts, rabbits can sustain temperatures of -20 degrees Fahrenheit. Cover up holes in barns or cover the hutch so there is no direct draft on the rabbit. If you are breeding in the winter, you must take precautions for mothers and freezing kits..." -Domestic Rabbits
Nov/Dec 2012 Issue Pg. 11
1. Tarp up the rabbitry or close off places that will cause a draft.
2. If the night seems particularly cold, stuff their cages/hutch with hay. Rabbits will bury themselves in it.
3.Watch your water bottles and crocks for freezing! Sometimes filling them only half way will help prevent your water bottle from becoming an ice block.
4. If you'd like, give them a little extra feed and some alfalfa hay during the winter to give them a little extra insulation.;)