Ugh, blight is the worst. Try switching to a completely different spot in the garden, like nowhere near where tomatoes have been for years. Those resistant varieties, like 'Defiant' or 'Mountain Magic', actually work pretty well. Also, water the soil, not the leaves, and space them out more than you think so air can move around. Cleaning up all the old plants at the end of the season is a pain but it breaks the cycle.
Remember when blight wiped out my whole crop? @danielm48 is right about moving them, but I also had to stop saving my own seeds from year to year. That seemed to finally make a difference.