We camped many times this winter, including new years eve in the Brecon Beacons and February in Snowdonia.
One of the most important things, if not the most important thing you can do is insulate yourself from the ground. We do this with closed-cell foam roll mats on the floor (very cheap, light and you can buy them everywhere) and self-inflating mats on top of that. Your body heat just dissipates in a big inflatable mattress - it's like sleeping on a mass of cold air.
Then we have 4 season sleeping bags. They're expensive but they mean you can camp all year rather than just the best summer months. If you're a car camper you can get a bulky one which is a fraction of the price of lightweight compact hardcore ones. But even more economical is to get the £10 fleece liner from Tesco. They make a massive difference and are so lovely to sleep in - soft and stretchy. Even nice on their own when it's very warm.
Another good trick is to fill a water flask (the screw-top metal ones) with boiling water before you go to bed. Wrap it in a jumper or something and put it in the sleeping bag to warm it up before you get in. Then when you get in hug it, put it by your feet, hold it between your legs, etc. Just make sure you don't burn yourself because it will be very hot to start with.
We bought 4.5 tog socks to sleep in and walk around the tent in too. We call them our 'magic socks' 
. You can buy them online and in a few sports and outdoors shops. Heat Holders they're called. £4-£5.
Oh yes, and make sure the tent is very well ventilated. You might think that the more open the tent the colder it would be, but if you allow condensation to build up inside the temperature drops a lot.
And sleep with a comfortable wooly hat on. You lose a lot of heat from your head. Likewise do your mummy sleeping bag up snuggly around your head.
We do all of the above things and we sleep very comfortably even when it drops below freezing. All you need is a little preparation and you'll never be uncomfortable again 
.