One of the best cold-weather preparation recommendations for your home is to caulk up any cracks or gaps. Your doors are an excellent place to start. Install some weather-stripping door draft stoppers if there are obvious gaps between the bottom of your door and the door frame to assist in keeping the cold air out.
The caulking around your windows should then be checked. Re-apply the caulk if you see gaps or regions where the original caulk has worn away. To keep the heat inside your home, you can also buy transparent, insulating shrink film and attach it to the inside of your windows.