I don’t know where it started, but there is a wives tale out there 
that you need to cut the tops off your crepe myrtle to get them to flower properly. This is absolutely not true! Crepe myrtles will produce abundant flowers and be healthier if you let them be.
The reality is that they will tolerate malicious topping style pruning as long as they get full sun. The pom-pom like growth that comes out from this kind of pruning gives the illusion that there are more flowers. It is important to know that they must get full sun to tolerate the abuse of topping. If they only get sun half the day or live in the shade it is likely your crepe myrtle tree will die within a couple years if you top them every winter.
One thing I have learned through my years of experience and a little education is that no tree grows branches and leaves for the purpose of people to cut them off. Topping your crepe myrtle is stressful, but they will tolerate it if the growing conditions are very good.
I cannot stress enough that you shouldn’t top your crepe myrtles, but if you must here are a few tips. First, don’t cut them in the exact same spot every year (like in the picture near the top) or unsightly knuckles will develop from the wounding. 
Either leave an additional 18″ of stem up from where you cut last time or cut below where you cut last time. It’s good to alternate; some years cut lower and other years leave them taller.
Normal pruning of a crepe myrtle would be to remove dead limbs and prune as needed for clearance, such as roof or driveway clearance. They don’t need any other creative pruning techniques. If your main problems is that your tree is not flowering like you want, look at the growing conditions. Any crepe myrtle will produce abundant flowers without topping. 
The main two things to look for are sunlight and irrigation. Crepe myrtles demand full sun all day to produce robust flowers. You may need to prune back some adjacent trees to help get additional light.
If maintained properly crepe myrtles can be beautiful shade trees. There are many varieties of crepe myrtles ranging from dwarfs to tall shade trees growing 30′ or more. Rather than top a tree lower than it wants to be naturally, why not just plant something that wants to be the height you demand. There is always a better solution than topping.