If you'd like to know just when enough is enough in the shower, but you don't want to go the route of a system that just shuts off the water and leaves you with soap in your eye, or worse, surrounds you in claustrophobia-inducing inflated bubbles, then the more subtle waterpebble might be the solution for you. By monitoring how much water heads down the drain, it let's you know when you've crossed over into water-waster mode. Even better, rather than giving you a set amount to stay under, it puts you in competition with yourself for more efficient showering. According to Dezeen, you just put the tiny device at the bottom of your shower. The waterpebble measures and remembers the amount of water you consumed when you used it for the first time, and uses that as a bench mark for future showers. A series of LED lights tells you if you're being more or less water efficient.
It's also designed to be fully recyclable. The only thing to be extra careful of, of course, is stepping on it. The idea of stubbing your toe or slipping on this thing sitting at the drain of your shower is not exactly appealing.
You can buy a Waterpebble for about $5 plus shipping.
More on Saving Water in the Shower
Stop Whining and Give Yourself a Shower Allowance
Buy Green: Shower Head
Man Up and Save 57 Gallons of Water a Shower--Take a Navy Shower