To keep your lawn using the least amount of water, don't mow it short. Yeah, it may look better, but the shorter you mow your lawn, the easier it is for water to evaporate. In addition, short lawn has less plant to use in photosynthesis, this stresses the lawn and so it will want more water. I keep my lawn mower set on the highest setting.

Another trick is to water infrequently, but water deeply. Continual, shallow watering of grass leads to roots that stay near the surface. If you water infrequently and deeply, that will force the roots deeper, and your lawn will need adapt to less frequent waterings. This also will help prevent weeds from gaining a foothold since they need a lot of water near the surface to get started.

The time to water your grass is when the leaves start curling, right before they get brown. When that happens, give the lawn a good deep watering. First put on 1/4" of water (use a plastic cup on your lawn to measure the amount of water hitting the grass). Then wait an hour. Then put on another 1/4". This small delay between waters will give the soil time to loosen up so that the second watering gets deeper into the soil.