It only takes a few grains of sand between your teeth to ruin an otherwise fabulous salad. Lettuces and leafy greens are great at trapping soil in their leaves as they grow, so shortly before you make a salad or sauté some greens, wash the leaves as many times as it takes to remove the grit.
You need enough water for the leaves to float freely. Depending on how many leaves you're washing, fill either a large bowl or a disinfected sink with cool water. Add the separated greens, swish them around to loosen the grit, and then let them float undisturbed for a few minutes while the grit settles to the bottom. Lift the greens out of the water into a clean container, leaving the grit behind. Drain the sink or bowl and repeat with fresh water until no trace of grit remains. Don't pour the greens from the bowl into a colander: you'll just pour the grit back over the greens.
Water droplets clinging to cooking greens are fine and actually help steam the greens during cooking, but salad greens need to be dried thoroughly. (Wet greens will dilute your dressing, and they won't last as long if you're not using them right away.) The only really effective way to dry them is in a spinner. To store, pack the leaves loosely in a zip-top bag lined with paper towels, gently squeeze out most of the air, and seal.