You need to move your tomatoes to a new location each year.  Once the spores of the disease are in the soil they will remain and reinfest the next year’s crop.  You should wait 2 years before planting there again.  Black or red plastic mulch covering the soil will also help prevent the spores from splashing up onto the leaves…this is how the diseases get their start.  Also NEVER spray your tomatoes with water.  Wet leaves are the perfect place for disease to take hold.  Cutting off the leaves closest to the soil will also help.