If you buy a four-pack or six-pack of transplants from a garden center, it's a good idea to transplant them to individual pots and harden them off for a week or two before setting them out in the garden. They'll have a more vigorous root system and you can make sure that the soil is warm and the weather settled before planting day.
Except in extremely hot climates, plant tomatoes where they will get full sun. To lessen shock, though, transplant seedlings on a cloudy day. Make the planting holes larger than normal for each seedling; cover the bottom of the hole with several inches of sifted compost mixed with a handful of bonemeal. For magnesium, which promotes plant vitality and productivity, sprinkle 1 teaspoon of Epsom salts into each hole. Disturb the soil around seedling roots as little as possible when you set them in contact with the compost.
Set the transplant so the lowest set of leaves is at soil level; fill the hole with a mixture of compost and soil. Or you can bury the stem horizontally in a shallow trench so that only the top leaves show; make sure you strip off the leaves along the part of the stem that will be buried. Many growers claim this planting method produces higher yields. Press down the soil gently but firmly to remove air pockets, and water well.
If you're planting a bit early, or in general want to speed the growth of your tomatoes, you can shelter them with a commercial device such as a Wall O' Water or simply wrap tomato cages with clear plastic.
Spacing between planting holes depends on how you grow your tomatoes. If you're going to stake and prune the plants or train them on trellises, space the seedlings 2 feet apart. If you plan to let them sprawl, space them 3 to 4 feet apart.
Letting plants sprawl involves less work, but it requires more garden space. And unless protected by a very thick mulch, the plants and fruits are also more subject to insects and diseases due to contact with the soil—not to mention being more accessible to four-legged predators, such as voles.
If you plan to train your tomato plants on stakes or in cages, install the supports before planting. Pound 5- to 7-foot-long stakes 6 to 8 inches in the ground or insert the cages (it's a good idea to secure cages with stakes, too). As the vines grow on staked tomatoes, tie them loosely to the stake at 6-inch intervals with soft twine or strips of cloth or panty hose.
There are also ready-made tomato cages, but they are expensive to buy and usually aren't tall enough. For details on making your own tomato cages, see "Super Sturdy Tomato Cages".
Any slight frost will harm young tomato plants, and nighttime temperatures below 55°F will prevent fruit from setting. In case of a late frost, protect transplants with cloches or hotcaps, because cold damage early in a tomato's life can reduce fruit production for the entire season.