There's an old saying that you can "trim trees whenever your saw is sharp." This is more or less true, but how you go about trimming them is crucial, both to ensure you don't come to harm in the process and to improve the tree's overall health. A chainsaw is dangerous in unfamiliar hands and should be treated with the respect it deserves. It is also one of the most effective all-around lawn care tools a homeowner is likely to possess.
Assess the tree to be trimmed. Having some foreknowledge of how you want it to look when you've finished is crucial. Ideally, the tree should be aesthetically pleasing and symmetrical. Identify any dead branches or limbs which break this symmetry. Keep in mind that by cutting off blooms, it will be a year or more before the tree develops new blooms.
Put on your protective clothing, boots, ear plugs and eyewear. Your clothing should all fit snugly to keep a loose sleeve from catching in the saw's teeth. Your boots should be steel capped to protect your toes if the chainsaw were to be dropped
Start your chainsaw according to the directions of the user's manual, and begin with your first tree limb. The first cut must be on the underside of the branch, roughly 5 inches away from the main trunk. It should extend no further than one-third of the way through the branch, as the blade will be pinched by the sagging limb otherwise.
Retract the saw, place it on the top of the branch several inches further away from the trunk than the previous cut, and saw down. Again, the cut should not extend more than one-third into the tree limb. This is called an "overcut" or "safety cut." It will keep the limb from splitting under its own weight. This protects the tree from damage, as well as keeps you from having a tree limb lash back at you should it break unexpectedly.
Apply the third cut from the top of the limb. This cut should saw all the way down to meet with the first cut, severing the tree limb.
Saw through the remaining stub of the branch sticking from the trunk for cleanliness. No safety cut need be used; a simple downward cut will suffice.
Turn off the saw, set it aside, and clear away the fallen tree limb. This will prevent you from tripping over it with a live chainsaw in your hands later. Once the ground is cleared, start up the chainsaw and continue cutting branches until the tree is free of dead branches and symmetrical.