Rotate, rotate, rotate. That’s the key preventive measure for trying to reduce the repeat of tomato diseases, many of which do in fact overwinter in or on top of the soil (such as in debris left behind from faded plants). A multi-year rotation of tomato planting spots is the best possible idea, but even a single-year rotation is better than none. Of course how many of us have three or four ideal spots for our tomatoes? There are so many tomato diseases, some of them fungal (meaning spores definitely can be left behind), other bacterial (again, bacterium can be left behind on volunteer plants or elsewhere)…the list goes on. So rotation is your best prevention. Do also add compost and such to build up the bed. And only grow a non-tomato relative there in alternate years, a non-solanaceous plant (so no peppers or potatoes). Cruciferous veggies or salads are a great alternate choice.