Pruning a crepe myrtle is a relatively easy process. You don't need any special tools or skill to do the job properly. You do need to ensure that you tackle this job at the right time of year, however. Unlike the other plants in your garden, your crepe myrtle should be pruned in the spring or early summer. Pruning in the fall or winter leaves your plant exposed to cold temperatures that can kill it. You will likely need sharp garden shears as well as large loppers to complete the pruning job. Also wear thick gardening gloves to protect your hands from sharp stems. If your tools are sharp, the job will go quicker.
Many varieties of crepe myrtle are susceptible to powdery mildew. By removing dense materials from the center of your plant, you will increase air circulation and reduce the risk of mildew. Begin by looking inside your plant for dead limbs and remove them as close to the trunk as you can. Next, look for branches that cross one another or rub together and remove them. Finally, remove branches that turn inward and remove them. All branches should point outward. Leave the thick, heavy stalks as they are, but trim away any sucker branches that are growing from the roots.
If your crepe myrtle has been neglected for several years, you may have more work to do. Determine how tall you want your crepemyrtle to be, and cut branches back to that size, as close to a bud as you can get. This sort of trimming will force the plant to grow thicker branches, although you may not have as many blossoms in the year in which you do a severe pruning. This is, again, another reason to prune your plants in the spring, when the buds have formed, as it will be easier for you to tell where to make your cuts.