1. Choose the Right Tool
A standard garden hose and nozzle is the least efficient means of applying water to plants because so much water is lost as mist, runoff and evaporation. Use a soaker hose or a sprinkler wand.
2. Don't Over-Water
For most Americans, a good rule of thumb is that a lawn needs 1 inch of water a week and perennial plants and shrubs will need from 1 inch to 2 inches a week. There's no neat rule for watering annuals, so your best guide is always the plant tag (the small spear-shaped plastic tag that came with the plant when you bought it). It will tell you the sun, soil, pH and water requirements.
Read more: 9 Water-Conserving Tips for Summer Gardening - Popular Mechanics