Only water your lawn or garden when it needs it. I know... surely everyone knows that! But how many times have you seen a sprinkler system merrily spitting out water in the rain? Or how many times have you gone to the store "for just a minute" and left the water on the lawn— for 2 hours?
Water deeply to promote root growth. Allow soil to dry slightly between waterings. Strong root systems create hardier turfgrass. Shallow waterings evaporate before water is able to saturate the soil where it's needed.
Mow your lawn at the recommended height for your type of turfgrass. Mowing grass too close causes stress and invites disease. Letting grass grow too tall also increases the chance of disease.
Remove thatch and aerate as needed to increase the soil's water absorption.
If you have an underground sprinkler system, keep it adjusted and well-maintained.
Don't forget end-of-the-year maintenance to protect your hoses and watering system from possible winter freeze damage. Water expands as it freezes. At the end of the season drain hoses, water timers and sprinklers before the first frost. Store them where they won't freeze.
Only water your lawn or garden when it needs it.
Most lawns need about 1" of water a week.
Fill the kiddie pool, but when it's time to empty it, use the water for flowers or garden, or a dry spot on the lawn. Or wash the dog in it.
Never water lawn or garden in the afternoon heat. Much of the water will just evaporate instead of soaking in.
Drip irrigation is more efficient for vegetable or flower gardens. Least efficient is sprinklers that spray far into the air.

You'll need the proper equipment to water your lawn and garden:
A quality multiple ply, reinforced garden hose.
A plastic or galvanized watering can with a detachable rose sprinkler head.
A water or spray wand that attaches to your garden hose for watering flowers and container plants.
Pulsating or oscillating sprinklers for specialized watering.