Some perennial herbs such as chives form clumps; others such as thyme spread by runners. Both spreading- and clumping-type herbs can be dug and divided in early spring to make new plants.
Propagate woody herbs such as rosemary from stem cuttings. Less woody herbs such as mint, oregano, thyme, and basil will also root easily from cuttings.
Stems of some herbs form roots where they come into contact with the ground. You can help the process along by layering the stems. Bend a flexible stem to the soil, nick the base of it, apply rooting hormone, pin it to the soil, keep moist, and wait several months for roots to form.
Prevent weeds from growing in your herbs by using mulch. Organic mulches such as wood chips, cocoa bean shells, or pine needles are good to use. Apply a layer of mulch 2-4 inches deep, but keep it away from the crown of the herb plant. Mulch smothers weeds, prevents most weed seeds from germinating, and those that do grow are easier to pull. In addition, mulch conserves moisture, so you'll need to water less frequently.