Clip fresh basil leaves as needed for fresh use. This can start as soon as the leaves of the plant are large enough for use. Clip the stem where it extends from a two-leaf joint. A new stem and leaf will grow from this point. Continuing to clip and use the basil while it is young and tender keeps the leaves and stems from maturing and forming buds and blooms. Once the basil plant has flowered, the leaves have a bitter taste.
Harvest basil for drying when tender and before the plant buds or blooms. Harvesting the basil later in the day, after the morning dew has passed, reduces the moisture content of the basil. Avoid the heat of the day which could wilt the basil before it can be dried.
Clip the center stems from the plant even if they won't be used. This keeps the plant producing new shoots, which are usable, rather than transferring the energy to buds and blossoms, which ultimately reduce the flavor of the basil.
Unless you live in a climate where outdoor temperatures stay above freezing, place the plant in a pot and move it indoors for year-around use. The plants thrive indoors if supplied with adequate water and light. Maintain the same harvest procedures using the fresh stems and leaves, while preventing the plant from blooming. Start new seeds outdoors the next growing season.