Tomato plants rapidly grow and often double their size every 12 to 15 days. Once they begin to make more sugar than they can use (usually after the plant has 10 to 13 leaves and is 12 to 18 inches tall), the tomato plant begins to make new branches. It also begins to flower. If left to its own devices, it will grow into a tangled mess of stems and branches with low quality fruit. Pruning off selected leaves and stems encourages strong stems with more, larger fruit. Pruning allows the sugar to be directed to fruit production rather than stems and leaves.
Remove yellow and brown leaves from the tomato plant. Clip them off as close to the stem as possible with a scissors.
Remove leaves and stems below the lowest flower cluster. Cut them off at the main stem.
Thin out leaves so that they all receive ample sunlight. Leaves that are in full shade will use more sugar than they produce, which will reduce fruit production.
Clip off sucker stems and leaves that form between the leaflets and the main stem. If left to grow, these sucker stems and leaves will grow to produce more foliage and flowers. However, these will weaken the main stem of the plant and reduce the overall vigor of the tomato plant.