Pick off the small shoots that form just above the leaf on the main stems. If one lets them grow they will form suckers and take a significant amount of energy away from producing fruit. Try to keep one main stem and allow for lots of air and sunshine to penetrate into the interior of the plant.
(Source: Ask a Master Gardener, Answers to your Gardening Questions from the Master Gardeners of Ontario Inc., pp. 23-24. Photo credit: Amy Urquhart.)