I love trimming crape myrtle trees.  They're one of my favorite trees to trim because they can look so ugly when they get overgrown but I can make them look so pretty.  I like to rub their bark with my hand and exfoliate the loose bark until the tree is smooth throughout.  I get all of the dead wood out and take out the crossed or rubbing branches and raise the canopy to the desired level.  This is how you trim a crape myrtle if you ask me.  But I have been asked many times before to commit crape murder.  That is, to just hack it off at one height.  I try to talk anybody out of it when they suggest it but there has been times when they have insisted upon it and since it is their yard I do it because customer satisfaction trumps proper techniques.  But honestly, I have never really seen an instance where I thought that crape murder was a good idea.  Yes, they'll live... just not as well as they should have.  Love your trees!  ~Peace~