Keep doors shut upstairs to prevent heat moving to the top of the house.
Tuck curtains behind the radiators to direct heat into the room, rather than into the window.
Got an unused fireplace? I can feel a draft going up mine, you can get a chimney balloon that blocks the chimney, it has a couple of gaps to ensure it remains ventilated and will melt if you light a fire/turn on the gas fire. I just have an empty hearth so I don’t have the worry of keeping a gas appliance ventilated so am planning to use the cheap method, apparently you can use the foil inner from a wine box and inflate it in the chimney, with a bit of plastic pipe tucked in the corner to keep some airflow. (the proper thing costs about £25!)