From: DK Books - Herbs
Division by Cutting Up
Many small-leaved, spreading or matt-forming herbs can be easily multiplied by this method, which is best done in late spring or early summer when in active growth (Image 1).
Using sharp scissors, trim excess leafy growth from around the sides and lightly trim the top (Image 2). This is most easily done with the plant still in its pot. Compost the trimmings.
Carefully remove the pot (for washing and reuse) and slice or cut the root ball in half (Image 3). The bottom part can be discarded and composted as it is likely to be exhausted and root filled.
With this herb, the aim is to make plugs with a surface area of about 3/4 x 3/4 in (2 x 2 cm). By carefully cutting using scissors or a sharp knife it is possible to generate nine or ten good plugs (Image 4).
Fill and Label the Pots
Fill each pot with fine compost, firm down and make a hole with your finger about 3/4 in (2 cm) deep. Insert one plug in each hole, gently firm in place, and water well (Image 1).
Label each pot with the plant name and the propagation date (Image 2). Place in a bright, warm spot and keep moist. Results should be clearly visible within a couple of weeks.
Division With Forks
Carefully remove the herb from its pot or dig it up from the garden. Gently tease out congested roots and remove excess compost and soil (Image 1). Passing the clump under a running faucet can help.
Gently pull the clump apart with two kitchen forks, detaching sections of the herb, complete with their root systems (Image 2). Discard any portions that are diseased or damaged.
Avoid handling the root system and transplant at the same depth as they were originally (Image 3). Use containers of fresh, sharply draining compost mixed with vermiculite in equal parts.
Resuscitating Leggy Plants Before Division
Leggy plants need not be destroyed as they can be easily rescued to provide many new plants or one compact large one. Partially fill a clean, larger pot with compost and insert the herb (Image 1).
Cover the woody stems with compost up to the base of leaves. Gently firm and keep moist until new roots have developed. The plant can then be divided or its stems used for cuttings (Image 2).
Many pots of congested herb seedlings, such as basil, parsley, and chives, can be divided. The stems and roots can be very fragile, so break down the clumps into no more than half a dozen portions.