Make one of the twin stoppers (with a tube on each side) if you can - they stay on the door when it's opened and closed.
We just spent a very cold winter in an unheated, barely insulated house in New Zealand. Here is what I have learned:
- stop any draughts. Add foam insulation to the windows if there are any gaps at all that would let cold air in. Same with the doors.
- put up heavy curtains with thermal linings
- keep doors closed in any rooms you aren't using
- if you are doing any baking, leave the oven wide open once you've finished. The hot air will warm the kitchen

- get some warm slippers and woolly cardigans that can be thrown on for extra warmth
- get a hot water bottle and use it if you're sitting watching telly or similar - it'll keep you warm even if the room is chilly
Now that we are in our own house and not a rental, I am adding:
- make sure there is plenty of insulation in the ceiling
- make sure there is damp proofing and insulation between the ground and your floor
- consider temporary "double glazing" using shrink plastic, if you don't have/can't afford proper double glazing
Hope some of that helps!