A lot of people swear that we should prune tomatoes. The Indeterminate kind that produce through a season rather than over a short 2 or 3 week kind. This video from Fine Gardening Magazine (Sister to Fine Woodworking) by Tuton press gives you the basics of pruning. Even if you do not do pruning and let your Tomato Plants bush all over, you should pinch off all growth along the main stem of a foot or less above the ground. This helps prevent fungus growth from the ground up - although I have found it will not guarantee that you will stop it. Just copy and paste the link below.