Remove crossing or rubbings branches that can cause wounds where insects and disease can easily enter the wood. Remove narrow-crotch angles. Wider angles are stronger and provide better support and weather resistance. Remove suckers at the base. Suckers are quick-growing shoots that drain a plant's resources.
Aphids like to feed on suckers.
Avoid pollarding your crape myrtle. You pollard a tree when you cut the tree's main branches back to its trunk. Pollarding is used in some formal gardens but it's not advisable for the healthy home garden. Allow a crape myrtle tree to grow as a tree. A consulting rosarian will provide instructions.