Tomato plants will grow on the ground if you let them, causing a vine-like tangle and increased chance of mold and decay. Keeping tomato plants off the ground by staking and careful pruning will decrease the possibility of disease and increase production of quality fruit. Trimming the leaves will encourage the plant to grow straighter by adding strength to the main stalk.
Trim off lower leaves with pruning snips. Cut them off at the main stem. Lower leaves tend to be shaded and do not always produce fruit, which means they are drawing sugar away from the rest of the plant. Cutting them early in growth will help make the main stalk strong. Cut off leaves below the first flower stem.
Cut off any leaves that are turning yellow, so that they don't draw sugar away from fruit production.
Cut leaves above the flower stem if you want to concentrate on larger fruit. More leaves means more fruit, but since sugar will be drawn to all the leaves, this will mean smaller tomatoes.