All kinds of liquid based thing are now being sold in "pen" and "moist towelette" form. This packaging is small and rugged, making it easy to carry in your gear bags. Some of the most helpful I've seen include stain remover, suntan lotion, machine oil, insect repellent, and hand sanitizer. I'm sure there are more. It's often the little things that can save the day, so go out and grab several of these and add them to your camera cases, messenger bags, or glovebox.