Keep the bathroom door closed as much as possible. Keeping the door shut helps preserve the heat inside the room and prevents it from escaping into hallways.
Check the weather tightness of the bathroom window and replace it if it is not energy efficient. Replacing an old leaky indoor window with a new energy efficient one can save you money in the long run.
Cover as much of your bathroom tile or linoleum as possible, using a number of throw rugs and plush floor coverings. Use rugs designed for bathroom use; bathroom rugs typically include a rubberized material on the bottom to prevent slip and fall accidents. The rugs help to keep the floor warmer and preserve the heat in the room.
Install a small space heater in the bathroom. Turn the heater to its lowest setting for everyday use, then turn it up as needed when you take a shower or bath.