A little while ago I wrote a post about the Curtains stating that they are the best value for money energy efficiency measure. Well recently I visited a house that had thrown that out the window (no pun intended).
If your house has radiators under the windows like the one shown here, then take care that the curtains are not closed on the room side of them, trapping the heat they emit away from the room beside the window. This crazy set up happens a lot and means the heating system is mainly working to heat the earth, not your house. If your house has this problem, make sure to put the curtains between the radiators and the window, directing the heat into the room.