How to prune and train tomatoes up a string from http://www.beginner-gardening.com and award winning garden author Doug Green. There are several important things to understand about pruning tomatoes, the first is that they way a significant amount when fully loaded and the tomatoes are ripening.
Secondly, when pruning them up a string it is important to remove the suckers and only allow a single stem to grow up each string. I used to grow tomatoes commercially and this system works perfectly for both training the tomato all season and cleaning it up quickly in the fall.
Beginners sometimes ask what you attached the tomato to the string with? The answer is nothing. If you bend the tomato around the string a complete revolution every 6 to 8 inches, you will find the tomato will stick quite nicely all by itself by friction. You can see this in the video.
So feeding and watering tomato are critical steps to success but so is the way you train and prune it. The nice thing about a tomato is it grows so quickly that if you do manage to take out the wrong bits from the wrong piece of the wrong direction, it will regrow and give you a second or third chance.
I prune and thin my tomatoes in this way because pruning to a single stem, will give me more tomatoes per square-foot than if I let them flop on the ground.
Pruning tomatoes is easy once you get the hang of it. You can see other videos at http://www.douggreensgarden.com