"WHEN TO PRUNE Autumn to early spring.

TRAINING AND PRUNING Train as a single or multistemmed tree, selecting one to five strong shoots to form main stems arising at or near ground level. These will support a proliferation of slender, flowering laterals and sublaterals if desired. Once the framework is established, prune one in three of the laterals and sublaterals back to the framework. This stimulates new growth that ripens to bear flowers from mid- to late summer. Plants grown in frost-free climates need little further pruning. In cooler areas and under glass, especially in northern areas with short growing seasons, thin to allow light and air into the center of the tree to discourage disease."

So sayeth the book Pruning & Training (DK, 1996).

Here's another link too!

Here is a link that might be useful: Pruning Crepe Myrtle