Pinching your tomato plant to remove excess stem growth does several things. First, it helps the plant focus its energy and nutrients on fruit production. Second, it keeps the plant from growing too densely, which is one of the factors for common tomato disease and pest problems. Third, a multitude of side stems weaken the main stem and can cause your tomato plant to break. Your pinching strategy will vary depending on if your plants are determinate (i.e. bush tomatoes) or indeterminate (vine tomatoes).
Wait for the tomato plant to produce a flower cluster. This may take four to six weeks.
Pinch back any side stems that sprout below the first flower cluster. These branches will appear in the elbow of the plant where a leaf branch meets the main stem. Pinch and tear off the tip of the side stem, leaving its base intact with several leaves still on it.
Observe the tomato plant. More stems will eventually begin sprouting off of the pinched stem. Continually pinch back the new growth.
Wait for the tomato plant to produce a flower cluster.
Allow up to three side stems to grow, starting at the point where the first flower cluster appeared and no lower. The closer the side stems are to the first fruit cluster, the better. The further up the vine you allow side stems, the weaker these stems will be and the greater the chance of breakage.
Pinch back all other side stems. Grab the tip of any extra side stems and tear it off with your fingers. The plant will continually try to grow new stems, and vigilance is needed to keep these growths in check.