Mulching tomato plants serves many purposes. It helps retain water since it slows down evaporation and promotes water absorption. It also helps keep soil temperatures more suitable for growing tomato plants by either keeping it warm in cooler environments or cool in hotter environments. Adding mulch also prevents weeds from invading the planting site; and for tomato plants specifically, it helps the lower growing tomatoes from rotting on the soil.
Select your mulch. You can choose a mulch made from wood, such as bark, wood chips and even sawdust, which are suitable for all locations. However, if you live in a warmer climate (e.g. the South), you may want to consider choosing a heavy mulch, such as pine needles or straw because they better protect the roots from getting too hot. On the other hand, if you live in a cooler environment (e.g. New England), you may want to use black plastic sheeting as your mulch, which keeps the roots nice and warm, even during cold nights.
Prepare the planting site by first weeding the entire area around your tomato plants, if you haven't already. Then, if you live in a warmer climate and you are going to apply traditional mulch--not plastic--you can add soil around the stem of the plant that starts about 6 inches above the ground (at the stem) and then slopes downward and outward toward the ground. This will add another layer of insulation to keep your plants from getting too hot.
Add three inches of your chosen mulch--not the plastic--and cover about 2 feet around all sides of each tomato plant. For plastic mulch, use a 3- or 4-foot wide sheet and install it when right before you plant your plants and on soil that is already moist. Secure it with stakes every few inches so the wind will not catch it and rip it out of the ground. Get it as tight as possible. Then, cut holes or make slits every four feet and plant your tomato plants. In addition, add a few holes around each plant to allow extra rain water to get in.