Stay warm, dry, hydrated and well-fed.Each human body puts out beewten 70 and 500 BTU/hr of heat (depending on activity level and available calories). A decent tent will trap some of this heat, to take some of the edge off the cold, but not enough to substitute for the insulation provided by clothing and/or a sleeping bag. The use of type of heater inside a tent is discouraged for safety reasons. So to stay warm, you need an appropriated-rated sleeping and clothing system.(Note that each body also exhales about 8 ounces of water each day, and without adequate ventilation and tent design, this moisture will condense on cold tent surfaces and possibly run down walls to get sleeping bags and other gear wet.)On the sleeping end, your first concern should be a sleeping bag rated appropriately for the lowest temperature expected, with an additional 20*F margin. I.e., if you expect 20*F, your sleeping bag should be rated for 0*F. There's a few reasons for this: 1.) most sleeping bag ratings are optimistic, and are seldom truly comfortable down at their minimum rating; 2.) it might get colder than predicted during your trip. If you don't have and can't afford a properly-rated bag, it is possible to improve a bag's cold weather performance by using a thermal sleeping bag liner, by doubling-up on sleeping bags (i.e., one inside the other), or by using additional blankets on top of the sleeping bag. In addition to the sleeping bag you will need a foam or insulating air/self-inflating mattress to protect you from the cold, hard ground.As for clothing, use layers of clothing that you can remove or add according to the temperature and activities. It is important to stay warm, but crucial to stay dry. Wet clothing does not insulate as well as dry clothing, although wool, silk and many synthetic materials still provide insulation value when wet. Cotton should be avoided as it becomes a heat conductor (rather than insulator) when wet, and takes a long time to dry. The basic layers are:1. On-Skin (underwear top bottom)2. Base Layer (long johns)3. Outer Layer (thick to thin, depending on activity conditions)4. Wind/Rain shellDon't forget essentials such as a hat and gloves. If the weather will be wet or you will be very active if would be worthwhile to bring an extra set of underwear, socks and base layer. Aside from the insulation, there are some things you should do to keep warm:1. Drink lots of fluids, particularly warm fluids. But avoid sweetened and/or caffeinated beverages. If your body gets dehydrated it will affect its ability to regulate temperature. Warm beverages help keep your body core temperature high without consuming calories.2. Eat extra calories, particularly fats. Your body will need extra calories to generate heat. Fat provides the ideal fuel for your internal furnace. In particular it is beneficial to eat a hot, high fat meal close to bedtime. 3. Stay active. The more you move the more heat your body generates. Be cautious not to overheat and sweat.Finally, if additional heat is needed you can fill water bottles with hot water, wrap in extra clothing and stuff them inside your sleeping bag. These will need to be refilled every 4 hours or so. You can also use chemical hand warmers that will last 8 to 12 hours.