Curtains = Cosiness!

Drawing your curtains when it gets cold and dark outside helps enormously to keep the heat from escaping through the windows. Windows, even double glazed ones, lose a lot of heat compared to what is lost through the walls. 

Invest in some thicker curtains, or line your existing curtains with thermal linings. Thermal blackout lining is not very expensive if bought by the metre from a fabric outlet. Or you can use polyester fleece fabric. 

Curtains are most effective at keeping in the heat if they are floor length, actually touching the floor, to stop warm air travelling up underneath. Pelmets are good at keeping the air from travelling over the top and behind the curtain. 

You don't even have to just stick to windows - fixing a curtain over the front and back doors can help to stop draughts too. 

And if you are really posh, you can fix curtains around your four-poster bed to keep you toasty warm all through the night! But perhaps that is going too far.....!