Okay. First thing is: wait! Don't prune your crape myrtles (Lagerstroemia spp.) (By the way: it's crape not crepe- they're not pastries) until after the new year, February is ideal. If you prune too early the wound left by the cut won't heal as it would during active growth: water and disease can enter and could cause that branch to die back several inches. You could be left with ugly dead sticks at every place you made a cut. Also, pruning too early can cause premature growth during the random warm spells that are not uncommon during the winter in this area; the new growth will then be damaged and probably killed once temperatures return to 'normal'. Don't wait too long though, have your crapes pruned by the Ides of March (the 15th): you want to have the pruning done before the new growth starts in the spring.
Second thing: Don't commit "Crape Murder". There is no reason to whack your crape myrtles back to a certain height every year. I repeat: THERE IS NO REASON TO DO THIS. Despite popular opinion, crape myrtles don't have to be pruned at all! *Gasp!* It's shocking, I know, but they are trees and can be left to grow as such. The reasons that we prune crape myrtles are 1. because our neighbors do it. 2. to concentrate those fabulous blooms into tighter more showy clusters and 3. to keep a cleaner look, as crapes like to sucker and look a little messy. - First, choose your tree carefully if you're thinking of putting one in. There are some cultivars, like 'Natchez', that will grow to 30'+, and then there are shrub forms, like 'Victor', that will stay under 5'. Like needing the right tool for the job, you need the right plant for the space.
- A good rule of thumb is to not remove more than a third of your tree's height.
When you make a cut to remove a branch, make it just above the branch collar (the little wrinkle at the base of the branch where it attaches to the trunk) pruning above or below this can damage the tree. When you make a cut to shape the canopy you will not prune to a branch, just cut it at the height you want on the branch at an angle so water doesn't sit on it.- Remove any branches smaller than your pinky finger, these branches will not support the weight of a bloom cluster.
- You should be pruning 8-12" above the cut you made last year; cutting in the same place makes a big knot of wood at the top of a thick trunk with teeny tiny branches coming out of it: it looks ridiculous.
- Remove any branches growing from the outside edges back into the center, these can rub on other branches and cause wounds that allow disease to enter.
- The finished product should look like a snow cone: V shaped with a rounded dome; the branches in the middle cut higher than the ones at the edges.
- Please see the photos, courtesy of Rick Smith at The Pruning Guru.
Now, raise your right hand and read these words outloud:
"I (your name) do solemnly swear that I will not butcher my Crape Myrtles. I will do one of the following three things: I will follow these directions, I will hire a competent professional, or I will leave them alone. With liberty and justice for all."
Now your correctly pruned crapes will be on their way to being happy, healthy and giving you beautiful blooms to enjoy all summer!
Jessica Logan - Horticulture Technician, Chattahoochee Technical College
Second thing: Don't commit "Crape Murder". There is no reason to whack your crape myrtles back to a certain height every year. I repeat: THERE IS NO REASON TO DO THIS. Despite popular opinion, crape myrtles don't have to be pruned at all! *Gasp!* It's shocking, I know, but they are trees and can be left to grow as such. The reasons that we prune crape myrtles are 1. because our neighbors do it. 2. to concentrate those fabulous blooms into tighter more showy clusters and 3. to keep a cleaner look, as crapes like to sucker and look a little messy. 
When you make a cut to remove a branch, make it just above the branch collar (the little wrinkle at the base of the branch where it attaches to the trunk) pruning above or below this can damage the tree. When you make a cut to shape the canopy you will not prune to a branch, just cut it at the height you want on the branch at an angle so water doesn't sit on it.