Rule 23: Always keep your head on a swivel.
It can apply to just about every situation. I've heard this saying a lot, but I have no idea about it's true origins. I've heard it most often when used either in connection with football or the military, but it applies perfectly to both. It's all about awareness for the sake of safety.
A swivel is nothing but the connection point between two things that allows each side to freely spin or rotate. The rule just dictates that your head should figuratively be able to spin on your body so that you can always see your full surroundings.
Keeping your head on a swivel is not about paranoia. Instead, it's about being aware and prepared at all times [not just on the football field or battle field] so that you're ready for anything that comes at you.