The primary consideration when selecting a mummy bag is its temperature rating. Most bags are divided into two categories: three-season and winter. Three-season bags generally have ratings that range from about 45 to 20 degrees Fahrenheit. Winter bags can keep you warm in temperatures as low as minus 15 degrees. Find a sleeping bag that is rated a little colder than the temperature you're going to camp in. Take into consideration your "sleeping temperature." If you tend to be cold when you sleep, buy a bag rated at least another 5 degrees colder.
Down or Synthetic?
The filling for sleeping bags comes in two varieties. Down, with its unrivaled warmth-to-weight ratio, often packs into a small ball and is the best choice for ultralight backpacking or really cold destinations. It's also more expensive, and getting a down mummy bag wet renders it useless. By contrast, less-expensive synthetic filling will work fine when it's wet, but it's heavier and won't pack down as small.
Sleeping bags work by trapping a boundary layer of air around you and letting your body heat keep this layer warm. It's important to keep this heated layer from escaping by sealing the mummy bag using its zippers and cinches. Keeping your head covered is crucial, as this is the area where the body loses most of its heat. Your sleeping bag also needs to be insulated from the ground with a sleeping pad.
Mummy bags are meant to be packed into stuff sacks, which are usually supplied with the bag. Slide the main zipper of the bag about three-quarters open, then stuff it into the sack. If your bag is down, keep it away from liquids and protect it in a waterproof bag (or garbage bag in a pinch) if it rains.
Care of Your Sleeping Bag
Sleeping bags should be stored dry and loose, not in the stuff sack. They also need to be washed periodically, preferably by hand, with mild soap or specially formulated sleeping bag wash. Help keep the bag clean on the outside by using a clean pad, and on the inside by wearing extra layers to bed or by using a sleeping bag liner. Note that both of these options will add to the warmth rating of the bag.