Don’t be a fool! Trim wisely.
If branches are bothering you it can be tempting to “snip them back a little”. Far too many people go ahead and cut right where they want the branch to stop, or where they can reach easily. Unfortunately these aren’t the best points to base your pruning decisions off.
Always cut near a union or healthy bud (That gives the energy and hormones being sent to the piece removed a good place to go). Don’t cut just anywhere on a stem or branch.
Don’t just snip tips unless you want a hedge! Many plants respond to having a tip of a branch removed by growing two or more new shoots from near that spot as well as sending more energy to the shoots/buds lower on the branch. If you want a thick plant like a in hedge this is great. If you are trimming a maple back from your deck, not so much. Thoughtfully removing a long branch and leaving a smaller one to fill the void is much better than snipping the ends back constantly.
Conifers need special care when pruning! Removing too much of a branch on many of these will cause it to die back to the next union, which is often the trunk. Obviously this results in a rather ugly dead branch. There are times when it simply isn’t practical to change the shape of a mature spruce or pine beyond removing complete branches.
Cutting the top off of mature trees is also strongly discouraged as the tree will be unable to heal properly, and is likely to respond by growing multiple new tops with weak attachment points. Don’t “top” trees!
If you need to trim, make sure you have a fair idea how the tree will react. Make good pruning cuts, and if you have a lot of trimming to do, spread it over a few seasons so the tree has time to heal and adjust.
If you are interested in learning more about the how and why of these tips, a good place to start is with learning about apical dominance… and then looking for answers to all the questions that you’ll have after that. Here is a link for wikipedia’s article. https://en.wikipedia.org/wiki/Apical_dominance