Trimming or pruning a maple tree, like with all trees, encourages healthy growth. Although maples are notoriously low maintenance, a healthy tree can easily outlive many other tree or plant species, as long as certain guidelines are adhered to. Pruning the sides and bottom branches is fairly intuitive, but the maple tree's top needs to be addressed as well. There are several domestic varieties of maple in the U.S., all requiring the same approach to pruning.
Set your stepladder up in early summer, a time when the maple trees are fully leafed. Doing so later in the season or early fall will slow down the growth process as well as leak sap, which doesn't harm the tree but could create a sticky mess. Make sure the ladder is stable by placing ladder shoes underneath the legs and have someone hold the ladder for you.
Brace the top of the ladder against an adjacent branch that is as close to the tree top as possible. Use a reciprocal saw to saw away any smaller vertical branches that inhibit access to the tree's central peak. Although maple trees have roundish plumes, there are often times a central "leader" branch toward the middle that acts as a guideline for other branches' growth. Indentify this branch.
Saw this branch at a 45-degree angle cut until the branch is the desired height, which is a matter of taste or to create enough of a buffer between the tree top and objects like power lines. Trim all the remaining branches on the top in this manner. You can trim the surrounding branches to the same height to level off the tree top or slightly lower than the central leader to create a slight "peaked" look.
Move your ladder around the perimeter of the tree to gain access to the branches and cut a more uniform look to the maple tree. Dispose of the cut branches, either as mulch or dried kindling. Following the tree's next dormant cycle, the branches will follow the "example" of the central leader and grow to its height.