Water deeply and infrequently to encourage deep, more drought tolerant roots. Only water when absolutely necessary! Turfgrass should be watered to a depth of at least 6-8” while flowers and vegetable gardens should be watered to a depth of at least 8-12”. Trees and shrubs need to be watered to a depth of 12-18” as that is where the bulk of their roots are located. Watering much deeper than these depths is a waste of water. Check watering depth by using a probe of some sort (i.e. long screwdriver, piece of rebar, strong wire, etc.). When the probe hits resistance you have likely hit dry soil and this is the depth to which the moisture has reached.
Drip or sub-surface irrigation is the most efficient method as it places the water exactly where you need it. If watering with sprinkler systems or by hand, irrigate early in the morning to take advantage of lower wind speeds (hopefully), less evaporation, and higher humidity levels. Watering at this time of day also allows plant leaves to dry off quickly, lessening the threat of potential disease problems.