"You should start by identifying and counting your main plant stems.  For any tomato variety besides cherries, you should aim for 6 to 8 main stems; however, if it's late in the season and you have more stems than that firmly established, you can refrain from butchering your plants--there's always next season to aim for that 6 to 8 number we recommend.  For cherry tomato plant varieties, you can have as many as 12 to 14 main stems. The next step is learning how to identify what are commonly called "suckers."  These are officially called apical meristematic plant tissue.  You can begin pruning your suckers once you have 6 to 8 main stems already established (12 to 14 for cherry tomatoes).  Here is a photo of a sucker, or apical meristem." - Todd Hoff from Bookmarklet