Herbs are at their peak of flavor when they've just been harvested. Midmorning, just after the sun has dried the leaves but before it's too hot, is the best time of day to harvest. You can cut back as much as 3/4 of the plant without harming it. If the leaves are dirty, rinse them quickly in cold water and dry them with paper towels.

The best time of year to harvest is before the flowers bloom. You can prolong the optimum harvest time by pruning off the flower buds as they form, but even the most attentive bud-nipper will find that the harvest season will eventually come to an end, and it's time to think about preserving some of your crop.

Freezing or preserving in olive oil are the best ways to preserve herbs, but many gardeners prefer drying because it is easy and doesn't require any special storage.

Here's a tip to keep the dust and bugs out of your herbs while they are drying:

Hang your bundles of herbs in a cool airy place, and place a paper bag over them. Make sure your herbs are tied in bundles small enough that they don't touch the sides of the bag when they are covered, and tie the end of the bag closed. Check back in about three weeks. When the herbs are dry they will crumble easily into jars for storage.

Be sure to also read:
All About Herbs