When growing tomatoes in an upside-down planter, it's important to keep them pruned. Because upside-down tomato plants aren't staked or caged, their main stem must be strong enough to support the weight of the fruit as it develops. Allowing too many side shoots and suckers to grow depletes the main stem of energy and weighs down the plant.
Hang the potted upside-down tomato plant by suspending the top of planter from the hook on the planter's stand. The seedling should be poking out of the planter's base and pointing toward the ground. (As these devices vary, see the instruction manual for more detailed directions related to your specific planter before you hang it.) Wait for the first batch of flowers to form.
Use gardening shears to cut away all side stems that have grown in above the flowers; they will not produce fruit. Snip the stems at the joint where their base meets the main stem.
Pinch all suckers off the tomato plant. Suckers are thin branches that sprout in the joints between the main stem and side stems. They rob the main stem of food, but frequently become too hidden in the plant's leaves to participate in photosynthesis. To pinch off the suckers, grasp the sprouts between your thumb and index finger at their base. Squeeze the suckers until they pop off of the stem. If the suckers are more mature, you may need to cut them away from the joint with a pair of gardening shears.
Continue to remove the suckers as the upside-down tomato plant grows, checking it daily for new sprouts.
Trim away any side stems that begin to brush the ground, particularly if they are not producing fruit.