Every tomato plant is one of two types: determinate or indeterminate. The seed packet tells you which type of plant you have, or the information is on the tag that comes in a potted tomato plant. Determinate tomatoes should not be pruned, or you will be removing potential fruit-bearing limbs. Indeterminate tomato vines need pruning to produce the best fruit and stay healthy.
Remove Lower Branches
Prune off the lower branches of an indeterminate tomato plant. Anything below the first (lowest) bract of blossoms or fruit can be removed. This helps the plant form a sturdy "trunk" and reduces the possibility of soil-borne diseases splashing up onto the lower foliage from rainfall or irrigation.
Check at Leaf Nodes
Every leaf joint where a bract of leaves is connected to the main stem grows a sucker, or side shoot. Look in the "V" where the joint is; if there is a small bud of leaves beginning to grow, simply nip it off with your fingers. Larger suckers look like small branches growing from the joint. Grasp them and firmly snap them off. If the sucker bends and does not snap off, snip it with clean pruners.
Prevent Spread of Disease
Snapping or pinching out suckers is preferred over using pruning clippers. Any tool used from one tomato plant to another increases the risk of spreading disease. If you prefer using pruners, keep a small bucket of disinfecting rinse handy so you can dip the tool between snips. Use one gallon of water with one teaspoon bleach in the dip bucket.
As you prune away the superfluous tomato vegetation, tie the growing plants to supports. Although tomato plants are vines, they do not have tendrils or any means of supporting themselves. Soft cloth strips or soft twine are good for tying tomatoes. First, tie the twine around the support post, leaving long ends free. Then take the ends of the twine and loosely tie a loop around the vine. This keeps the vine from rubbing against the support and allows it room to grow.
Work When Plants are Dry
Any time you work around tomato plants, they should be dry. Fungal diseases and soil-borne pathogens are easily transmitted when the ground or foliage is wet. Wilt diseases and some fungus diseases are incurable, so prevention is the best option.