From the Voice of Experience—The Oregon State University Extension Master Gardeners

Use an application rate that permits water to soak in rather than run off from the garden area.

Concentrate your watering in the root zone. The less water you apply between plants where the roots haven’t yet extended, the less water you lose to evaporation, and the fewer weeds you are likely to have.

 
 
Drip irrigation or soaker hose systems (low flow) are worth the investment as they promote the best possible growth and are great for conserving water.  They can be set up to precisely meet individual plant watering needs, and then adjusted as the watering needs change throughout the growing season.   Additionally, you can combine drip emitters, soaker tubing, and miniature sprayers and sprinklers for an effective and efficient watering system.  Low-flow systems, when compared to conventional methods of watering by hand or with sprinklers, can save up to 50% water.

Sprinklers can waste water by watering the unnecessary areas of the garden, and with unnecessary runoff, which can be reduced by watering at a slower rate.  Additionally, water on plant foliage can lead to diseases.  You can gain efficiency by watering during the cool, wind-free time of day, and also placing your sprinklers so that the tall plants aren’t blocking the sprinkler spray from other areas needing water.   Also check the patterns of your sprinklers to make sure all necessary areas are covered while avoiding the unnecessary areas as much as possible.  Check your sprinkler heads each season as well.  It sounds like an overwhelming and technical task, but it’s really not that difficult to do. 

Drip emitters can be placed further away in clay soil than sandy soil because water spreads out more horizontally in clay soil than in sandy soil. 

Hand watering is an effective and efficient method of watering since you can control exactly how much water and where it goes, however it is time consuming.  It always good to spot check the areas as well to make sure the soil is at the recommended dampness/dryness level for the plant type.