It's a simple matter but if you leave a door open longer than it needs to be, you are losing heat to the outside.  When it's really cold, heat that escapes will need to be replaced and your heater is probably running frequently as it is.  When leaving your home, be certain you have everything in hand and can quickly open and close the door.  Same thing when entering; shut the door without delay.  You save some money and the comfort level in your home will be maintained.