There is also a type of rechargeable hand warmer, which doesn't last as long, but may be recharged many times by placing it in boiling water. I often use neoprene gloves. I have found them in sporting goods stores where they are often grouped with sport fishing accessories. While they won't keep your hands warm in real cold weather, they are flexible enough to allow one to operate a camera easily. Some people prefer gloves which have slits in them, allowing the ends of fingers or thumbs to be exposed and used.
Cold film is brittle.
Use care when loading the film, as leader ends may crack, or sprocket holes may break. If you are planning to stay outside for a long time, bring along some extra batteries. Spare battery holders, preloaded with fresh, warm batteries can make life lots easier in severe cold. Once photographic equipment is cold, as long as it keeps functioning, don't bring it back into a warm spot unless you are prepared to leave it there until it completely warms up. Fogged lenses make foggy photos.