Trimming, or pruning, tomato plants involves not only removing leaves, but also removing the small branches, referred to as suckers, that grow in the "Y" between a branch and the main stem. Trim mature tomato plants towards the end of the season, usually when tomatoes outgrow their support. Trim tomato plants at this stage to direct growth energy into the existing fruit instead of the developing new fruit.
Pinch suckers off by hand or with a pair of clean, sharp garden shears. Remove suckers at their base where they meet the branch.
Remove any spent foliage. Look for wilted or yellow leaves. Trim just the leaves at the base of the leaf.
Water plants with a gentle stream of water at soil level to protect the plant's open wounds.
Check plants daily for new suckers or dying foliage.