You can staple or sew cheap fleece onto the back of regular curtains for additional thermal insulation.
hang curtains so they go behind radiators or sit on windowsills, rather than falling prettily in front of them to the floor, so the radiators heat the gap behind them not the room.
Draught excluders - stuff old tights with something heavy - sand, rice, kapok, whatever you've got - and put them at the foot of each door.
You can get a film to put over windows if they aren't double glazed.
Keep doors closed to any unheated rooms, and if the garage has a door into the house consider hanging an extra thick curtain with it too.
Rugs on floors, blankets on chairs, blankets under you as well as over you in bed.
You can get letterboxes with springs on them which stay closed, or you could seal it up altogether and go for an external letterbox with a lock and key.
There's insulating tape you can put into door frames which catches breezes nicely.
We have a shedload of candles in our sitting room which make that the warmest room in the house in the evenings.
And once we've finished cooking with the oven, I always leave the door open as it cools down; releases the heat back into the kitchen rather than leaving it behind glass. On a similar note if it's particularly cold, I'll cook things by boiling them on the hob, as we seem to get more heat generated that way than by using the very well insulated oven.
Best of all though, consider getting cats. They'll find the warmest spots for you, and they'll act as animated hot water bottles whenever you sit down. Just make sure the cat flap is draught proof!