Turn down whatever heating you use around the house and put on extra warm clothing.

You would be surprised how few people do this!

And a tip or two:

1. on a cold night wear a hat or balaclava indoors (most body heat goes out through the top of your head!)
2. …and buy some nice thick skiwear socks.

If you have a heater on, don’t make the room go above 21 degrees. You pay a lot more if you do.