Space/insulation between you and the cold floor! Inflatable mats that will lift you off the floor and insulate are massively helpful. You could combine this with a reflective foam mat.
Take a hot water bottle, use camping stove to boil water.
Very good sleeping bag. Do not rely on over optimistic "3 season!" Or whatever claims. Go for one that is really designed for cold weather.
Wooly hat.
Breathing into the sleeping bag can help warm it up a lot. Spend a few mins with the top pulled up like a chrysalis after you get in, exhaling into it, to warm up the air inside.
Hand warmers for extreme conditions! You can get rechargable usb ones. Could put one for hands and one for feet.
A single layer of thermal long johns and long sleeve top. You don't want to prevent your body heat warming the sleeping bag because you are wearing too many layers. However you can go mad with an inner sheet for sleeping bag, extra blankets etc.
If you are two, creating one sleeping bag by zipping them together will be much warmer. Sweet.