"If it's yellow, let it mellow," is a nice sentiment for saving water. But let's face it -- it's pretty gross to use a toilet that hasn't been emptied. Not to mention the yellow ring it leaves that you have to scrub off later. If you want to minimize your water usage, swap out your toilet for a low-flow model that lets you regulate the amount of water needed for each job. It's also a good habit to make sure you haven't left any remnants on the toilet seat.