In order for tomatoes to grow successfully, they must have an even water supply. Watering tomato plants erratically can be as damaging to the fruit as over- or under-watering them. Tomato plants without a steady water source are at risk of developing diseases such as blossom end rot, which generally render the fruit inedible.
Water tomato plants in the early morning, before the sun has reached its midday height. This will give the moist soil time to warm up before the temperatures drop in the evening, making sure the plants aren't sitting in sopping dirt overnight.
Apply the water to the base of the plant with a watering can or hose, being careful not to wet down the leaves and fruit as this can damage the tomatoes.
Make sure all water is absorbed into the soil. Tomato plants do need to be watered thoroughly, but take your time to ensure that the water isn't puddling on top of the dirt. If this happens, the plant has been over-watered.
Keep the tomatoes on a schedule. Water them every day at the same time if you live in a very hot area, or every other day if the heat doesn't cause the soil to dry out within 24 hours.
Plant potted tomatoes in pots with large, well-spaced drainage holes in the bottom. The holes will allow the excess water to flow out of the bottom of the pot. If the water is trapped in the pot, it can rot the plant.