During the winter, it is a good idea to weatherproof your windows. Weatherproofing your windows is an excellent way to keep the heat in and the cold air out of your home. It is easy to do (home weatherproofing kits can be purchased at many larger retailers) and it will save you money on your heating bill.
As well, throughout the winter, instead of turning up the thermostat in your Denver apartments, use a small portable heater to heat the room that you spend the most time in. The heater will help to reduce the monthly costs associated with heating the whole apartment and the less heat you use, the lower your winter heating bills.