You've got a problem called blossom end rot. It's caused by a calcium deficiency and it shows up the most when the plants are watered infrequently. First, make sure your tomatoes are getting a good soaking once a week. Mulch them with organic mulch like shredded leaves or shredded cypress or something similar. The mulch will conserve moisture between waterings. Finally, you might consider adding calcium to the soil. It is available at garden centers in liquid and powder forms. Bone meal is an excellent source of calcium. You can also use crushed eggshells left over from your morning omelet.