I've been grafting my heirloom and open-pollinated tomatoes onto vigorous hybrid rootstocks. This gives me all the great flavor and variety of the hybrids, but with much better disease resistance, more production and generally larger tomatoes at the end of the season when my heirloom tomatoes used to start giving me beefsteaks that were half the size they were at the start of the season. It takes some time, but when you have high disease pressure and a long season it pays off. At least that is what I've found in my garden.