Wash herbs when you are ready to use them. Wash smaller amounts of herbs thoroughly under running water. Shake off moisture or spin dry in a salad spinner. Pat off any remaining moisture with clean paper towels.
If you're washing a larger amount of herbs at one time, treat them as you would salad greens. Place in a clean sink or deep bowl filled with cold water and swish around. Lift from the water and transfer to another bowl so dirt and grit remain in the water. Pour out the water and repeat the washing process in clean water until dirt and grit are gone and the water is clear.
NOTE: If you plan to harvest a large amount of herbs from a home garden, consider washing them down with a hose the day before to help remove any large particles of dirt or grit that might be on the leaves.
Annual herbs can be harvested down to about four inches tall and they still will regrow for use later in the season. For perennial herbs, don't take off more than a third of the plant at any given time.