While Basil, Borage, Bee Balm, Marigold are all beneficial to the deterrence of the worm, they are long term deterrents. They are fragrant and companions to tomatoes. Fill your tomato areas with the complimentary and natural insecticidal plants.
In the meantime (aka NOW):
1. Inspect your every morning and evening for worms. They like tender leaves and the bore into the tomato at the stem. Pick them off immediately, then squash them, feed them to the chickens or, drown them in hot soapy water. If there is one, then there are more.
2. Spread diatomaceous earth around the base of your plants and on the actual plants. When it rains you may need to reapply. Be careful not to breathe it in and wear a mask if you aren't in an open area. It is like a powder and breathing any solid particles into your lungs should always be avoided. Make sure not to use pool grade or diatomaceous earth with any poisons added.
3. Contemplate the introduction of insects that feed on the worm: beetles, green lacewigs, paper wasps and braconid wasps.
4. Further consider the application of a chemical: Bacillus Thuringiensis (aka Bt). This is a natural bacteria that needs to be applied on the top and underside of the leaves. It is poison to the worm which kills them when they eat it. An organic product called Green StepTM Caterpillar Control (http://www.gardensalive.com/product.asp?pn=8797&sid=100308&eid) works great on tomato worms because it contains Bt. An effective all natural liquid concentrate that paralyzes caterpillars resulting in their death from starvation. You will notice crop damage to cease almost immediately. Green Step Caterpillar Control only affects targeted insects. Apply Green Step to the top and bottom of tomato plant foliage at the first sight of tomato worm damage. Then they suggest reapplying every 5-7 days. This is another highly effective way of controlling tomato worms.
Till soil in the fall and spring.
Wrap toilet paper rolls around the tomato stem at the ground.
Place a layer of wax paper around the tomato plant that is 3 inches below the surface of the ground.