I recently received a call from a friend asking if it was possible to extend blooming of crape Myrtle trees.  I replied with a yes.  All you need to do is simply remove spent flowers as soon as they finishing blooming.  Prune them off just above the first set of leaves behind the spent flowers.  I did this to my daughter’s tree about 2 weeks ago, and new growth is showing.  She just reported that new buds are starting to develop; normally it takes about a month or more before new flowers will appear.  Give it a try.