1. A good sleeping mat. If weight is not a consideration (as in you’re driving to your campsite, not walking), there are inflatable pads with very high R values. My Exped DownMat weighs about 1.5 pounds, has an R value of 7 and packs very small. I pair it with a closed cell foam mat (Thermarest) between the ground and inflatable for extra insulation.
2. As previously stated, your sleeping bag’s “rated” temperature is misleading. There are actually three ratings: The survival temperature (the one advertised prominently on the tag); you’ll live, but you’ll shiver a lot. The comfort rating, which is typically 15-20 degrees higher, where you’ll be happy; and a number in between those two, where you may feel somewhat cold, but not uncomfortably so. For outside temps of 5-10 degrees Fahrenheit, I’d have a minus 10 degree bag to stay reasonably warm.
You can fudge that a bit by layering a warm blanket(s) inside your sleeping bag. Don’t place it on top of the sleeping bag, as you’ll compress the down/poly fill and negate it’s effectiveness. Think of making a cocoon, with you in the center.
3. Warm, layered sleeping clothes, with a moisture wicking layer against your skin. Never use cotton as the base layer, as it retains moisture which may make you colder. Merino wool is great, as either a base or middle layer, with warmer fleece garments as outer layers. When selecting wool garments, ALWAYS read the label to see exactly how much wool is in the fabric. Socks/hats/gloves should be at least 50% and other items at least 90% wool for the best warmth. I prefer separate sleeping clothes, as whatever you’ve worn all day may retain moisture and salt from sweating, as well as being dirty.
3. The suggestions of Hot Hands and hot water in a Nalgene bottle are excellent. Add a warm hat or balaclava. Bonus points if your sleeping bag has a built-in hood.
4. Around camp, a fire will certainly help keep you warm. Again, layered clothing with an outer layer of a down jacket. If it’s windy, you can put a loose hardshell jacket over the down jacket. Wool gloves under wind resistant mittens. A balaclava and a warm beanie hat. When washing dishes, use hot water and wear rubber dish gloves, preferably with a thin pair of wool gloves as a base layer.
5. Allegedly, having a high protein, high fat snack shortly before bed increases your metabolism and raises your body heat. At least you won’t be hungry. Also, if you have to pee in the middle of the night, do it (not in the bed, lol!). Having a full bladder makes it difficult to relax and sleep well.