If you often walk among your herbs, you know there's a time in their growth cycle when they are heaviest with fragrance. For basil, it's before it sets flowers. For thyme, it's after. Oregano blossoms are as deliciously aromatic and flavorful as the leaves, so pick them when the mood strikes you. For parsley, the time to pick is before cool autumn weather toughens the leaves.
Mints are at their best before flowering, so pick then.
Harvest oregano and thyme when the flowers are barely open. Rosemary leaves are easy to dry. Pick them whenever you want. Pick sage flowers when the top blossoms are barely open. Wait another month or so to snip leaves.