I never kill the tomato worm on my tomatoes because they always have white little rice-like eggs on them. A parasitic wasp lays them, and they will kill the caterpillar. So - try never to kill one that has the eggs on it. When I find them, I take them off the tomato and put them on a plant in the edge of the woods.
Those things eat like crazy. My mom had a potted tomato plant on the porch last summer and the whole thing was destroyed almost overnight by a couple of Manduca caterpillars.