The right way to prune crape myrtle shrubs is best relayed not by textbook horticulture, but actually seeing it done, getting your hands dirty, exposing yourself to danger in some cases and having an artistic plan for the outcome, considering what will happen when new growth occurs. Hence, I am somewhat stifled in writing this article in accurately describing the "right" way...bear with me.
Picture your crape myrtle in a condition where you want to "do something" with is - thin, spindly growths prodruding in all directions, crossing one another, and potentially growing taller or wider than you would prefer. Now picture the "after" photo, consisting of 2 or more "Y" shapes where the miscellaneous growth has been shaved back to close to the trunk (but not too close to the trunk). Your goal is to cut off the limbs with a good clean cut to where that place will "heal" over with new skin (bark), showing that smooth bump that well pruned crape's have. Picture a clean set of "Y" shapes, perhaps clipped or encouraged to be "straight" instead of windy.
Here is a picture of a tastefully, horticulturally correct way of shaping crape's:
In addition to having an eye for the right outcome, proper tools are also needed to achieve a good showing crape: a sharp set of hand clippers, possibly a pole pruner using the saw attachment, possibly a ladder (with someone holding it at the bottom), a sharp silky saw, etc.
Also of note is the fact that if left in a stagnant area, crape myrtles may develop scale or a black residue on the leaves. This black residue is left there by the presence of bugs who will respond to #1 planting the crape in more of an "airy" environment and #2 treating with any over the counter sprays that you can find at Lowes.
When in doubt, call a professional tree service or landscape maintenance service!...but please, oh please don't just hack down this beautiful specimen in just any old fashion - we all have to look at that thing for the rest of the year as we drive by!