The Crape Myrtle Massacre is the over-pruning and wrong-season pruning of crapes all over Houston.
You've seen over-pruning ... people chop back crape myrtles to the knuckles each and every year, and new growth comes out like a "feather duster." And wrong-season pruning would mean November and December. Don't let "peer pressure" by neighbors and commercial gardening crews get to you.
Looking through all the horticultural research I could find, nowhere could I locate anything about trimming crapes in November or December. The reason is simple ... we don't have much of a winter. If you trim the crapes in the last two months of the year, and we get a warming trend in January or February, the trees might actually start putting on new growth. And that new growth will be incredibly susceptible to freezing weather should it come on the heals of a warm spell.
New growth will also tend to draw the cold right into the plant, causing needless damage to a tree that should be resting in dormancy.
So, say it with me: the best time to trim crapes is January through February. For years, I've suggested Valentine's Day is a great time to trim them, because at that time we're also trimming back our roses.
Now, as for how much to trim, it truly is up to you. Since crapes are so resilient, no matter how much or how little you trim really doesn't matter. From an aesthetic point of few, however, I personally don't like trimming back to the "knuckles." But you should at least trim back the expired seedpods (the dried bloom clusters) to insure better blooms in the coming year.