Tomato plants put forth a great deal of growth during the season, and they are heavy feeders. Tomatoes respond well to fertilizer applications, and if you use a correctly balanced fertilizer your tomato plants will produce fruit readily. Too much nitrogen will force tomatoes to grow lots of leaves and not many blossoms or fruits.
Prune away the lowest two or three levels of leaves on tomato plants when you transplant them into the garden. These are tender, and can be nipped off with your fingers, leaving several leaves above ground for the tomato plant to grow. You should plant tomatoes at least deep enough to cover the stem this far up, which creates plenty of opportunity for the tomato stem to develop a deep root system. Roots will grow all along the stem wherever it is in contact with the soil.
Prune tomato leaves from the lower stem as you tie the plant up to a stake, cage, or trellis. Remove the lower leaves that are below the first set of blossoms. This can help control wilts and other soil borne diseases that are spread by splash-up from rainfall or contact with the soil. It also produces a strong main stem. To prune leaves from the main stem, clip the leaf stalk about one-fourth inch from the main stem. The small stub will heal over or drop off in a few days.
Prune out leafy suckers. Suckers are leaf shoots that grow at the point where the leaves are joined to the main stem. Depending on the tomato variety, suckers can sap the plant's energy away from developing fruits. On some varieties, the suckers will also bloom and set fruits, and you will want to keep some of them. Plan the overall structure of your tomato plants when you prune. Prune small suckers by pinching them out with your fingers. Larger suckers can be snipped out with sharp clippers. Allow the branch of leaves to remain on the main stem when you prune out suckers.
Leave plenty of foliage for photosynthesis. Tomato plants use the sugars from photosynthesis to produce more foliage, and more and bigger fruits. Tomatoes need plenty of leaves to carry on the process and produce good fruits.
Leave plenty of foliage to protect fruits from sunscald. In the hot summer as fruits ripen on the vine, the direct sun can burn the fruits. They need some shading from the foliage to prevent this.