It' been a little chilly here these past couple days.
Well... if you think 0 is chilly.
When the weather turns like this remember the tips you used during the summer to keep you house cool - just reverse them.
Open your blinds/curtains during the day, especially on the south side to take advantage of the sun. As dusk arrives close the blinds and pull the curtains to retain the heat. Remember, if you have baseboard heat, curtains should not cover them. That kinda defeats the purpose.
If you have a draft under your door - there are plenty of options: install a door sweep, or a new threshold; get one of those draft dodger stuffed things that look like long dogs or cats; or heck... just toss a rolled towel there when it gets this cold.
I would not recommend having a fire unless you have glass doors. While you may feel comfortably warm in front of your sensual flames - most of your dearly paid for hot air in the rest of the house is going up and out your chimney.
And finally - remember the best (and least expensive) way to keep warm is to share body heat.