Both determinate and indeterminate tomato types grow suckers. However, determinate types are genetically programmed to know when to stop making foliage and start producing flowers and fruit. Indeterminate types, if left alone, would produce much more foliage than fruit. By pruning suckers, you are telling your indeterminate tomato plant to channel its energies (and sugars) into making fruit, not foliage. Luckily, pruning suckers is easily done as long as you are observant. You do not even need special tools for the job; in fact, tools can harm your tomato plants.
Watch your indeterminate tomato plants carefully. Observe the branches coming off the main stem in the center of the plant, as well as the smaller branches that will inevitably form between the branches and the main stem.
Pinch off one of those smaller branches between your thumb and forefinger, right at the base; it is a sucker. Bend it backward and forward gently until it pops off between your fingers. Do this when suckers are 2 inches or less long, if possible. If you let them grow longer, they take more energy away from tomato production.
Repeat this pruning whenever you see suckers appear. Stop them sooner rather than later to encourage growth of tomato flowers and fruit. Prune any branches below where the first flower clusters appear as well, also using the pinching method.
Discard pruned suckers in your compost heap, if you have one.