Knowing a few things about harvesting herbs will help you to capture their best flavors and aromas.
Herbs should be harvested when essential oils are at their peak. By harvesting herbs first thing in the morning, you'll capture their essential oils -- the quality that gives herbs their flavor and fragrance -- before the sun's heat dries it up.
Mature culinary herb plants should be pinched or cut back frequently so you'll get a steady supply of tender branch tips. If you use your herbs often, you're automatically doing this. Pinching also prevents herbs from flowering and setting seeds. This gives plants a second growth of leaves that will provide you with fresh herbs in autumn and winter.
Use sharp scissors when harvesting herbs with woody stems. Tearing at stems will leave ragged edges that can harm the plant and attract disease. Herbs with soft stems can be pinched off.
Cut just above a node -- the small bump or joint where a stem branches out. New growth is produced from nodes, so by cutting them here, you'll encourage new growth and branching. Even if you just want the leaves, cut the whole stem before stripping away the leaves. Remember to cut long stems if you're planning to hang the herbs to dry.
How much should you harvest? Allow perennial herbs to grow during the first year before heavy harvesting. Trim them lightly to promote branching of stems so you'll get a fuller, bushier plant. Once they're established, you can take up to half of the plant at a time without weakening it.
Since you'll only get one season of growth from annual herbs, you'll want to harvest as much as you can during the growing season. The same is true for biennial herbs, such as parsley.