Caulk smaller gaps. Most homes have cracks and leaks that are equal to a 2'x2' open window. For large cracks without moving parts, such as in walls, floors or foundations, apply spray foam insulation. Install storm windows or double layer windows. An economical storm window alternative is plastic sheeting.
Many types are available.
Look for those which can be heated with a hair dryer and shrunk to create a clear, non-distorted view. Use drapes to insulate. Keep drapes closed to keep warm air in. Keep them open to allow the sun's ray's to help warm chilled rooms. At Work Switch to energy efficient compact fluorescent lamps.