Tomatoes that are late to ripen usually are overfed and overwatered (much to the chagrin of well-intentioned gardeners). Once the vines reach the size you want, cut back on the fertilizer unless the plants show symptoms of deficiency (such as pale or stunted foliage, or misshapen fruits). Reducing water, even to the point where a little stress (slight wilting) shows before you water again, can push the plant to ripen its fruit. Watering this way also will stop more tomatoes from setting, which is desirable in areas that have short seasons, because the late ones won't have time to ripen. If your season is long, you may wish to water enough to keep more tomatoes setting on the plant, but doing so will slow the others' ripening.