While a properly-staked tomato plant, one that is kept up off the ground, generally requires only mild pruning, the way you manicure your tomato can have a great deal to do with both the size and the number of tomatoes the plant ultimately produces. Keep in mind that each tomato leaf's job is to turn sunlight and carbon dioxide into sugar. If a leaf is blocked from receiving sunlight it cannot do its job and, ultimately, turns yellow and falls off the plant. Proper manicuring can keep the majority of leaves producing sugar and making baskets of red, ripe tomatoes until the very end of the growing season.
Cut all branches and leaves below the first flower cluster on young tomato plants. Provide your tomato plants with a wire cage or some type of trellis support to hold its soon-to-be-fruit-laden branches off the ground. Be prepared with ties to attach branches to the cage or other support.
Cut off all dead or dying branches or branches that are limp or wilted. Remove all yellow or brown leaves.
Manicure branches that block the sun from reaching large clusters of interior leaves--but only if the branches you are cutting do not have any flowers or flower buds.
Prune cautiously. In most cases, only manicure dead, dying or diseased branches, and remove any yellow leaves. The primary way to maintain a healthy tomato crop is to keep your vines off the ground. That means tying them to a tomato cage or other support.