The Golden Rule is "Do unto others as you would have them do unto you." It's hard to apply this to tree trimming — you don't want your tree to prune you back! However, there are four different Golden Rules that apply to tree trimming, and if you want your trees to thrive, you should follow them.
#1: Always prune for a reason.
You should never just prune a tree for no reason at all. While you often hear that trees need to be pruned just a year, this is just an estimate. If you look at the tree and it's the right shape, all the branches are healthy, and there are no overly thick areas where branches are crossing, then you can simply skip pruning it that year. Taking healthy growth from a tree for no reason at all tends to do more harm than good.
#2: Don't take more than 25%.
Say your tree does need a good re-shaping. You still need to be careful how much you remove. The second Golden Rule is never to remove more than 25% of a tree's branches at once. And after you do so, wait a year before trimming again. (At this point, you can remove another 25% if needed.) If you remove more than this, the tree might not have enough leaves left to capture sunlight and make food for itself.
#3: Remove the whole branch.
Don't just clip off the ends of the branches! If you do, you'll be removing the part of the branch that is actively growing. The rest of the branch may just wither and die. If a branch is damaged, in the way, or otherwise undesirable, then remove that entire branch. Leave a little nub near the trunk, but nothing more.
#4: Use clean equipment.
Trees can get diseases, too. And trimming them with clean equipment can help prevent that. You need to sanitize your shears or trimmers before you use them. Use either bleach water or alcohol. This will kill any fungi or bacteria on the shears which could otherwise work their way into the wood and lead to disease.
These four Golden Rules of tree trimming will help ensure your trees get the most out of their trimming sessions. Do your best to adhere to them, and if you need any additional guidance, reach out to a tree trimming company in your area.