You basically has three options: heating, reducing area to be heated, and better insulation. Outside of this, there are no lifehacks ever going to save you, so let us tackle each one.
The most common heat sources are wood, gas and electricity. Electricity is by far the most efficient way in terms of heat produced. To reduce electricity usage, do use a heater directly instead of indirect heat through a refrigerator, lights, or other electrical equipment. They might have other benefits, but the heat value will be reduced as compared to a proper heater.
Gas can be a good alternative, but requires enough ventialation not to be a safety hazard. However there does exist good gas ovens, which are allowed for indoor use, and if you have the money you can invest in dedicated gas ovens for indoor use.
Wood is also a good alternative, and can be very useful and cheap if you have a good supply. This does however require a working fireplace, which the OP in this case says is not available.
Reduce area to be heated
If you have limited heating capabilities, it is always an option to reduce the are which needs to be heated. In one apartment I rented, I chose to close off two rooms in the winter, to reduce the electricity needed for heating.
Either close off rooms for the cold seasons, or accept that some of the rooms are going to be colder. You can also try to divide existing rooms using either light walls, or blankets in a pinch, which will help a little.
A variant of reducing area to be heated, is to allow the room to be colder, but use more and better clothes. This allows for you not to freeze, whilst the overall temperature in the room is lower. This can also be accomplished by using blankets when seated, or possibly a mattres heater when in bed.
Your best option is usually to get better insulation. Insulate walls, roof, floor and windows. Try to locate the coldest surface, and start there when insulating.
Note that adding blankets/rugs can help insulate both walls and floors. If you have really old windows, it can help with thicker curtains or if really cold by inserting extra windows or even insulation boards.
On a lighter note: A person usually generates around 100 W of heat, so inviting a lot of friends over will increase the temperature quite fast!