As I learned in the marines, you need to shave every morning so that your gas mask seals. Who amongst us hasn't needed it while touring?

Other tips, use a brush and a stick because it is the only solution which will not freeze in the arctic, nor go off in the desert. Use the hot water in which you heated your breakfast ration pack.

Finally, if you have a field inspection, clean the razor with your toothbrush after you use it [Edit: yes that is disgusting!]. Otherwise you might find yourself crawling for it. Been there done that...