To get vigorous tomato plants sow seeds indoors up to eight weeks before the last killing frost is expected in your region and transplant the seedlings at least twice before they go outdoors. Each time you transplant, set the seedling deeper in the soil -- from half to two- thirds of the way up the stem. Because tomatoes root easily from the stem, you get the plant to send out additional roots wherever the stem is buried.
Do the same thing when you finally set the plants outdoors: leave no more than a third of the plant above ground, stripping away the leaves from that part of the stem which will be buried.
When I set out my tomato plants I dig a shallow trench rather than a deep hole. This way I can bury most of the stem in the warmer surface layers of the soil by laying the plant in the trench. As a result, there is no setback from cold soil temperatures at deeper levels and the plant grows vigorously right from the start.
Once the soil has thoroughly warmed up tomatoes benefit from a good mulching (4 to 6 inches) with straw, shredded leaves, or straight compost.