Pruning tomato plants in a home garden is not a necessity, but rather a gardener's prerogative. Pruning will decrease your total tomato yield, but will help increase the size and taste of individual tomatoes. Determinate tomato varieties never need pruning as they only grow to a certain size and bear a certain amount of fruit. Unless tamed, indeterminate varieties will grow constantly throughout the growing season, producing shoots and fruit until the first hard frost. Pruning certain stems will help the vine support the weight of the fruit, and help keep it from falling to the ground.
Prune off side shoots, also known as sucker stems, by pinching them off with your fingers, or by using sterilized pruning snips or a sterilized garden knife. Suckers will begin to grow once your plant has taken root and reach 8 to 12 inches in height.
Inspect your tomato plants daily and remove suckers every seven to 10 days or when suckers are 2-3 inches in length, to keep vital nutrients flowing to your fruit and not the leaves of the plant. Suckers that have grown larger than 3 inches will need to be cut with a knife or snips.
Pinch the top three inches off the main stem once the plant has reached the desired height, usually the top of the tomato cage or stake, to limit the plant's upward growth. Allowing the plant to get to tall and top-heavy will cause it to tip over with the weight of the fruit, possibly breaking the lead stem.
Cut the ends off each stem two to four weeks before the first frost so the final fruits will have time to ripen before the plant dies. Doing this will stop growth and new foliage development, focusing plant energy on the fruit.