Heating water or more accurately, keeping it heated, is an expensive process and especially unfriendly to the environment if you're not using solar hot water system. Electric and gas hot water systems account for up to 20% of a household's greenhouse gas emissions.
 
We generally set our hot water services to a temperature that's too hot to immerse our hands in. Instead, we add a ton of cold water to achieve the desired temperature - which is rather wasteful; not to mention a safety issue. 

Also, every time you turn off your hot water tap, there can be gallons of hot water still in the lines - the heat is wasted and your hot water service then needs to reheat the equivalent of the water lost. Additionally, the hotter the temperature of your service, the less working life it will have.
 
For each 10ºF reduction in water temperature, you can save between 3%–5% in energy costs! When we were on bottled gas, we certainly noticed a substantial saving just by dialing back the thermostat a couple of notches.

Other strategies for saving on hot water:

-  Use faucet aerators

- Install low flow shower heads - this is very cheap to do and will save incredible amounts of water. 

- Check the laggings on external pipes regularly and replace if necessary

- Buy a water heater insulation blanket that can help reduce heat loss (more suited to electric hot water systems) - another very cheap item

- Wherever possible, use cold water in food preparation and cooking. Heating water on your stovetop consumes less gas/electricity as your water heater is likely to go into a heating cycle whenever hot water is used. Better still, use a microwave to heat the water

- Use cold water for washing your hands - there are really no health benefits in using hot water as the temperatures that kill bacteria are also too high for human skin.

- These days, with the improvements in laundry products, there's little need to use hot water in washing cycles. We use cold water for wash and rinse cycles and haven't experienced any problems with getting our clothes clean - just be sure to use earth friendly detergents.

- If you're replacing your dishwasher soon; consider buying one with a in-built water heater, which will save energy.

- If you are in the market for a new gas hot water service, consider a tankless system. Tankless systems operate on an "on demand" basis; heating  water when it's required. Anywhere up to a third of the energy used  by a standard water heater is consumed by maintaining water at temperature in between uses.

Maybe you've considered solar hot water in the past, but it was too expensive? It might be worth your while to look into it again. Not only have prices generally dropped and the technology improved; but many governments also offer renewable energy rebates that can substantially offset the cost; in some cases making these systems comparable in price to a standard hot water service. By switching to solar hot water, you'll not only slash your household carbon emissions, but up to 75% of your water heating costs.