The amount that you water your established lawn depends on the climate, weather, grass species and soil composition. In normal conditions a good soaking twice a week in summer is adequate, but this will depend on what water restriction you have in your area. Training you lawn to survive on less water is more desirable and less expensive, if you have chosen a drought hardy variety of turf it should be able to survive on rainfall alone. Water in the early morning or late afternoon to ensure all of the precious water is used by the lawn and no evaporated.
Change the way you mow your lawn with the way your grass grows. In winter, when it doesn't grow much at all, mow every 3-4 weeks and cut it longer so that the grass has more leaves to photosynthesize. In summer mow up to once a week and gradually decrease the height as summer goes on, cut the lawn shorter than in winter to ensure that grass grows more out that up. This will help the roots to grow send out runners and close the gaps for potential weeds to grow. However, do not remove more than 1 third of the grass blade when mowing otherwise you will "scalp"; the lawn, revealing the soil, causing more evaporation and spaces for weeds to grow. If you increase your current mower height by half an inch, this will improve the root system and allow it to maintain density.
Replace or sharpen mower blades every 12 months to ensure a quality cut and that diseases will not affect your lawn from blunt blades.