1. Don't wait until your plants are embarrassingly rootbound. If you notice roots growing out of the bottom of your pot, assume it's past time to get a bigger pot.
2. If you waited too long, don't worry. Tomato plants are pretty forgiving. Try to unwind any roots that are circling the bottom, and plant the whole thing deeper in the next pot. New roots will grow from the stem and it should be fine.
3. If you don't want to transplant at all, try growing it in a bigger pot to start with. A plastic container that holds about 600 to 750 mL (three cups) should be big enough to get your seedlings to transplant day.
4. When watering your newly transplanted tomatoes, consider doing it outside or over a sink, instead of over the kitchen table. As water and dirt flow through your drainage holes, you will realize that nobody bright would do that...