Some types of tomato plants require more pruning than others. Tomatoes fall into two types: determinate and indeterminate. Determinate tomatoes have a preset number of stems and a rigid growth pattern; these tomatoes require little pruning and can be harmed by excessive pruning. Indeterminate tomato plants can produce any number of stems and can be pruned for shaping and to promote larger fruit. Trim tomatoes during the growing season, in summer months.
Trim off suckers that form in the crotch intersections between leaves and the plant stem. These suckers will eventually bear fruit if left on the plant, but they compromise the plant's vigor in the meantime. Clip off the suckers with anvil pruners. If you have determinate tomato plants, this is the only type of pruning you should do, since more trimming eliminates potential fruit. Gardeners with indeterminate tomatoes should perform additional trimming.
Allow an indeterminate to produce a second stem from the node above the first fruit. This allows your plant to produce more tomatoes. Trim away competing stems that grow from the nodes above other fruits.
Let the indeterminate plant develop a third stem from the second node about the first fruit, and a fourth stem in the same manner. According to Fine Gardening, keeping the stems close to the initial fruit prevents the branches from dominating the main stem.
Clip off shoots that develop from other areas of the plant, leaving only the stems that grow from the nodes above the first fruit.
Top both determinate and indeterminate tomato plants 30 days before the first frost date in your area, suggests Fine Gardening. To do this, cut off all developing shoots on your tomato plant. This directs the plant's entire energy into ripening the fruit and allows your tomatoes to ripen (or get close to ripening) on the plant before you must pick them.