Pruning tomato plants helps fruit production because it allows the plants to be more efficient. When a tomato plant is left unpruned, many of its leaves become enveloped in the bushy sides of the plant. They cannot reach the sunlight to participate in photosynthesis, but they are taking up some of the sugars produced by this process just the same. When those branches are trimmed away, the sugars can be used by the plant to develop larger, more numerous fruit .
Allow the tomato plant to grow unpruned until the first set of blossoms develops.
Use your fingers or a pair of gardening shears to remove all the side stems that have developed below the first set of blossoms. These side stems, which are long shoots that branch of the main stem, rob the main stem of nutrients and are likely to be covered by the higher leaves as the plant grows. They will be of no use to the photosynthetic process.
Remove all suckers from the plant by pinching them at their base. Suckers are tiny offshoots that develop in the joint between the main stem and a side stem. They will not produce fruit, but the leaves that develop on these suckers will rarely contribute to the plant's health and will crowd the fruit and side stems.
Continue to look for and pinch off suckers throughout the growing season. Suckers can develop overnight on tomato plants, so it's best to keep on top of this task for maximum fruit production.