Don’t go to bed cold. Do something before! A couple of jumping jacks or a short runt around the camp. While you’re at it wave your arms and smile.
Nalgene bottle with boiling water, add a sock over it and put it on your stomach, in your armpits, next to your feet or groin for more heat during the night.
Clean clothes. Don’t sleep in the clothes you had all day.
Pee before bed. A large volume of pee will make you cold and probably you have to go out in the middle of the night.
Eat something before going to bed. (Energy to warm your body).
Sleeping liner or long johns and a hat will help you stay warm and keep your sleeping bag cleaner. Long johns also prevent your thighs from glueing together after a long days hike.
Don’t dry your clothes in the sleeping bag. In the sleeping bag you want to rest and get energy for the next day.
If your sleeping bag is too large you will easily get cold because you still have to heat up all the space you don’t really use. If you already have one or borrowed from a friend try to put a strap around it and close off the area you don’t use.
If your sleeping bag is to narrow you will put pressure on the sides of your sleeping bag and therefore the isolation will be more compact in that area. This will make you cold because the air in the isolation is minimised.
Get away from the wind. If you sleep under the stars without a tent or shelter the wind can “eat” its way into the bag and you will be cold during time. If you have a bigger backpack you can use it as a wind blocker. I once slept with a drybag 40L over my lower body and pointing towards the wind – it’s a good wind blocker. Also look for a bivvy bag if you repeatedly sleep under the stars.
If it’s cold outside a sleeping bag with collar will help you a lot. It will stop the warm air from getting out and cold air getting in. Every time you move, the air moves.
If you know your sleeping mat can handle cold down towards 0 Celcius but it’s colder you can choose the ground you pitch your tent on more carefully and get away with some more. A cold rock would easily chill you down but if you see an area with higher grass or fluffy vegetation you can pitch your tent above it. When the area under your tent get flat it makes a lot of small airpockets that makes it better isolated.
Clean and dry skin makes everything more fun!