Spinach should be washed very well since the leaves and stems tend to collect sand and soil. Before washing, trim off the roots and separate the leaves. Place the spinach in a large bowl of tepid water and swish the leaves around with your hands as this will allow any dirt to become dislodged. Remove the leaves from the water, empty the bowl, refill with clean water and repeat this process until no dirt remains in the water (usually two to three times will do the trick). Do not leave spinach soaking in the water as water-soluble nutrients will leach into the water.
Spinach sold in bags has been pre-washed and only needs to be rinsed. If you are going to use it in a salad, dry it using a salad spinner or by shaking it in a colander.