As you may already know, from your own experience, tomatoes (and all members of the nightshade family: peppers, eggplants, potatoes) should not be grown in the same soil one year to the next--in order to prevent soil-borne diseases. For those of us with small gardens, this can be a problem; we simply do not have enough space to rotate our kitchen garden beds. Reading the excellent The Great Tomato Book by Sheila Buff, I learned the following, terrific organic tip:
"For home gardeners who don't have space for rotating, planting a cover crop can be very effective. Cover crops, also sometimes called green manure, are planted not necessarily to eat but to improve the soil...try planting any member of the Braxxica family in fall as soon as the tomato harvest is done. The roots of these cool-weather plants give off potent sulfur compounds that help control tomato diseases. In spring, till them in a couple of weeks before you plant your tomatoes. Good choice are leafy Brassicas such as mustard greens, kale, and collard. Your primary goal is to improve your soil by killing off disease microorganisms, but cover crops like these also give you delicious early-season greens."