Having cash on hand is always a good idea, especially when you can sort of "forget" that it's there until there's an emergency situation. Fold up a 50 or 100 dollar bill so it's very flat and tape it to the battery cover in the back of your cell phone. You'll pretty much always have it on hand. Don't have a removable battery? Most smartphones don't. Stick the folded bill between the phone and the protective case.