Also only heat the rooms you need and close the doors of unused rooms.

Curtains

During the day open your curtains to let light and heat in and in the evening close them to prevent heat from being lost through the windows (especially if you don’t have double glazing). Keeping that in mind make sure you don’t drape the curtains over the radiator, as this will funnel heat straight out of the windows.

Radiators

Keep all the radiators unrestricted by furniture, carpets or drapes.

By placing heat reflectors (foil panels) behind the radiators (these are attached to the outside walls) you reduce the amount of heat lost to the outside. This is especially useful in houses with little insulation.

Fireplace

By closing the (glass) doors in front of your fire place stops heat from being lost up the chimney. If you do not have doors, block up the chimney by using newspaper or cardboard. Also, close the fireplace damper when not in use.

Windows

By replacing your windows with double-glazing cuts heat loss through windows by 50%.

Also place draught strips around windows (and outside doors). Perhaps fit a flap across the letter-box preventing cold air seeping into the house. You can actually test for air leaks by holding a lit incense stick next to the window (or door, plumbing fixtures, electrical outlets, attic hatches, etc) and watching how the smoke travels. If it travels horizontally, you have located an air leak that may need caulking, sealing or weather stripping.