Why do I -LOVE- handwarmers so much? Well, they're great for shaking up before you goto sleep. You can toss them down in the bottom of your sleeping bag and keep your toes warm. Some people will put a nalgene with hot water in their sleeping bag. I like handwarmers more because you can toss them in your boots in the morning and let them warm up before you put them on.
For use time and time again you can grab the reusable hand warmers as well. Just bend the metal disk inside and they will turn hot for several hours. Once your done with them rather than throwing them away if you put them into boiling water for 2 minutes they go soft and are completely reusable again. Means you don't need to carry so many, reduces waste and means that you can keep them going a long long time with no worries of running out.