Keep weeds under control during the growing season. Weeds compete with plants for water, space and nutrients, so control them by either cultivating often or use a mulch to prevent their seeds from germinating. 
Mulches also help retain soil moisture and maintain even soil temperatures. This is especially important for tomatoes as their roots may be easily damaged when weeding, and this can lead to blossom end rot.
Keep plants well-watered during the growing season, especially during dry spells. Plants need about 1-2 inches of rain per week during the growing season. Use a rain gauge to check to see if you need to add water. It's best to water with a drip or trickle system that delivers water at low pressure at the soil level. If you water with overhead sprinklers, water early in the day so the foliage has time to dry off before evening, to minimize disease problems. Keep the soil moist but not saturated.
Monitor for pests and diseases. Check with your local Cooperative Extension Service for pest controls recommended for your area.