Pruning of Crape Myrtles is often misunderstood.  As with any pruning, you want to prune with a purpose.

Most wives usually grimace when they see their husbands walking out of the garage with a pair of loppers and hand shears. My gosh man, show some self control!!! Don't unnecessarily hack back the branches of this tree (Crape-Murder). Pruning, if any, should be done to accentuate the tree's natural beauty.

Younger Crepe Myrtles prefer to grow more like a bush.  They may be encouraged to grow tree-form by removing suckers, (small sprouts at the base), and lower branches while leaving three to five of the sturdier canes to develop as the main trunks.  Thin out the inner branches--removing crowded, crossing, and rubbing branches--to allow better air circulation through the canopy.  This can also help with powdery mildew.

Use a thinning cut (trimming a branch where it originates) to remove branches up to 3/4" diameter.  Make this cut slightly out from the collar (the raised area of bark around the branch at the crotch) so the wound will heal quickly and smoothly.  Remember the smooth natural flow of this tree trunk and bark are part of it's appeal just as the blooms are.

I know what you are thinking, "But if I don't chop it back it won't bloom!  I must chop and lop and snip, for I own these great clippers!"  Relax...remember self-control. 

Crape Myrtles bloom on current season growth, so the idea was that cutting them back encouraged more new growth, hence more blooms.  However if you thin out the previous mentioned growth, you shouldn't need to cut back all of the old flower clusters.  If you still must, just don't cut the stem back past wood that is pencil size in diameter.  They don't trim off the clusters of the twenty foot specimens at the Arboretum and they still bloom nicely.  Hey, we all like lower maintenance, right??

Do most pruning in late February to early March before new growth.  Minor trimming can be done during the growing season to balance the shape and drooping branches.