First know what is best for the species you are pruning/cutting off branches, or tree removal situations of safety first.
Second, know the best technique …for example a bald cypress tree needs specific methods…. different from an oak.
Tips on Cutting Back Trees…..Proceed branch by branch to determine what cuts are necessary. Cut back each overly-long branch to a branch fork with a green shoot growing from it. This is the most important rule for cutting back cypress trees: never cut all green shoots from any branch since the branch will not be able to grow more….general rules for different species vary. Proceed from the underside of the branches, slanting the cuts up. When you are pruning cypress trees, aim for a natural look by pruning some branches deeper into the foliage than others. Obviously, natural look goes out the window when trimming back from power lines at least three feet.
Third, have a bonded and insured tree trimming service like http://Lawns-Gardens-Landscaping.com do the job.