When I was getting ready to take the state board cosmetology exam, one of the items they made sure we carried with us was Alum, because it helps stop bleeding after a small cut.
The same applies in shaving - You get a small cut, you can use alum to stop the bleeding. Alum has natural antiseptic properties in it, which makes it perfect for those pesky shaving nicks. But powdered Alum isn't convenient or clean.
The Art of Shaving's Alum Block, on the other hand, is easy, quick and creates no mess at all. It looks like a bar of soap, and all you need to do is get it wet with cold water and hold it over your nick or just wipe it over your nick, depending on how much it's bleeding.
The Art of Shaving also recommends using it before using an electric razor, as it will help give a closer shave.
If you dry it off after use and store it in it's case, it will last forever. This is nice to have on hand for your teens, who are learning how to shave.
The Art of Shaving: Alum Block - (buy direct)