Watering Basics from Gardeners.com (with some additions)
Six Tips For Watering Your Garden
Focus on the root zone, as in, water low to the plant. Remember that it's the roots that need access to water, not the leaves. Wetting the foliage is a waste of water and can promote the spread of disease. Instead, feel the soil. If it doesn’t hold together you need to water.
Water only when needed. Watch the weather, and reduce frequency when rainfall is abundant. Too much moisture can be just as damaging to plants as too little. If a plant is wilting, check the soil before you assume it is going to die! It may bounce back at night. Regardless, saturate the soil when you are planting seedlings.
Some plants as more fussy about water than others. Check plant watering charts online if you are not having success keeping your plants healthy.
Water deeply and thoroughly. Annuals, like vegetables, concentrate their roots in the top 6" of soil. In heavy soil, it may take hours for water to percolate down 6-12". Use your finger or a shovel to check the progress.
Water in the morning. If you do get moisture on the leaves, this gives them time to dry out. It's much more difficult for plant diseases to get a foothold when the foliage is dry.
Mulch everything. Mulch reduces surface runoff and slows evaporation from the soil.