Pruning tomato plants thins them out allowing for air flow. Good air flow helps the plants to be healthier and fight off diseases such as blight.
Indeterminate tomates will keep growing so it's best to prune them
Look for shoots growing just above leaf branches
Snip off shoot close to stem and leaf branch
This shoot is so small you can easily break it off
Here's a big shoot. Notice the leaf branch below it
A pruned tomato plant
The before picture is at the top. I don't know why Cookie is so tired she hardly helped at all.