Know what's in your First Aid Kit. Beyond that, know how to use it.
For example, some of the kits sold by Store-It Foods contain Cold Spray, a non-medicated spray designed to cool skin fast in order to help relieve pain and swelling. This can be very important in the moments following injury. On the other hand, the manufacturer warns that it is highly flammable and should not be inhaled. As well, it is "not to be used in confined spaces and not to be used on the head, neck, fingers or toes." A further warning: "Avoid spraying near the eyes." These are the kinds of things to know BEFORE use.