from ucanr: Dry herbs just don’t compare to fresh herbs in cooking. But, while some flavor loss in all dried herbs occurs, your own dried herbs will be fresher and more pungent than any you can buy at the supermarket. Here are the basics:
Pick herbs for leaf harvest just before flowers open. This is when the leaves contain the highest concentration of oils. Leaves may be harvested until late summer. Seeds may be saved by allowing some or all plants to mature and flower, and harvesting when seeds change in color from green to brown or gray.
The day before harvesting leaves, spray herbs with water so that they will be clean and dry the day of harvest. The day of leaf harvest, pick herbs in the early morning or place stems in water for two hours after picking. Strip damaged lower leaves and remove any flowers (unless you are harvesting seeds from the flower heads as described below).