It's been mentioned already, but really is important, so I'll reiterate as my one tip. It's not exactly creative, but so important!
Ventilation. Ventilation. Ventilation. It makes sense in our minds to shut them up completely to lock in heat and keep out cold air and wind, but that will actually do them more harm than good. It might make sense in our minds that the eaves or ventilation slots across the top of the coop let out all the warm air since warm air rises, which is true, but as does the moisture, which you want to get rid of. Moisture equals frostbite equals cold chickens. If you've ever had frostbite.. you'll know to keep them vents open!