Be certain to heat water only when you need it. Think for a moment how this works. You have a large tank located somewhere in your home right now that contains hot water. When you are not at home during the day (possibly at work or even when you are sleeping) you are constantly and unnecessarily heating that large water tank. A tankless hot water heater, on the other hand, only supplies you with hot water when you need it, so the natural gas is off when the hot water is off. There is no tank in which to store hot water. This is one of the biggest "no-brainers" out there: Only use natural gas to heat the water when you need hot water.  This will cut the cost of your gas bill by at least 25% for the heating of your hot water.