we did several things, husband was a structural engineer. One room in a house was single skinned and with an uninsulated floor over a garage. Beyond freezing. He put wall studs up and created an inner skin and we put down a good carpet over floor insulation. Big improvement
Another house was single skin brick, he had another skin put over the outside with an air gap in between. Increased the loft insulation and we eventually bought better carpets
Basic things make a difference, loft insulation, insulating the gaps around doors, even cardboard under a carpet, door curtains, I put clingfilm on windows when we were skint and had metal frames with single glazed windows
I have also seen insulating wallpaper
Like urmstongran, I am very glad that I decided to move into my highly insulated new build three years ago.