A prepared tomato bed is one that is warm and ready to welcome your plants. Warm soil helps plants adapt quickly to the new environment, and makes nutrients more available to the roots allowing the plants to start growing more quickly.
First, pull out any weeds.
Second, cover the beds with 2ml clear plastic (visqueen). Secure with rocks around the edge to keep it in place. The plastic causes the covered soil to heat up faster and stay warmer than the surrounding soil. This works even better if your bed happens to be raised 6-12 inches as well.
On planting day, I remove the plastic and plant my tomatoes in warm cozy soil where their roots will grow and establish themselves without any delay. This is important, especially depending on the length of your growing season because in some areas, a delay could mean little or no tomato sauce for you.
Don't worry, if you didn't have a chance to cover your beds before planting time, plant your tomatoes anyway and keep notes on how they do. Then remember to cover your beds next year and you may find that your plants produce more quickly in the warmer soil.