Tomatoes need lots of sun, consistent watering and rich loamy soil. If your tomatoes aren't producing as well as they should, perhaps a change in location would increase their yield. Tomatoes already set with fruit are available from nurseries; look for tomatoes with fruit in 1-gallon or larger containers. Tomatoes need a strong root system for the strongest plants. According to the National Gardening Association it's best to transplant on a cloudy day.
Transplanting from the Garden
Dig the hole for the new location first. Since tomato roots reach down as far as 5 feet, make the hole at least 2 feet deep and wide. Amend the soil with compost, mulch and slow release fertilizer. Fill the hole with water and let it drain.
Water the tomato in its old location 24 hours before you transplant it. Remove any fruit that is ripe or beginning to ripen. Cut back the vines that don't have fruit by one-third. It's not possible to dig up all the roots; cutting back the vines will help reduce the stress from transplanting.
Dig up the tomato with as much root ball as possible. Start digging at least 12 inches away from the stem. Push the shovel straight down and gently lift up until you've dug a circle all around the tomato. When one side is loose, slide the old towel into the hole. Dig directly underneath the tomato plant, separating the plant from the soil. Slide the towel under the root ball and up the other side. Use the towel to carry the tomato to its new location.
Remove branches and leaves from the lowest part of the stem for 6 to 8 inches. Estimate how deep the root ball and the additional stem length is and backfill the hole. The buried stem will produce new roots. Lower the tomato into the hole with the towel. It may take two people, one holding each end of the towel. Remove the towel by sliding it out from underneath the root ball.
Fill the hole with soil. Tamp it down firmly around the stem. Water. Shade the tomato plant for a few days until it adjusts to its new location.
Transplanting From the Nursery Container
Dig the hole as above.
Remove the plant from the container. Prune the branches and leaves from the bottom 8 to 12 inches of the stem. If the plant is root bound, scratch the exposed roots with a knife to loosen them a bit.
Lay the plant in the hole so it is leaning sideways. The stem that you removed the leaves from should be under the soil level. Fill in the hole with soil.
Shade the plant if the weather is very warm for the first couple of days.