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Keep in mind different cultivars have different growth habits. I also recall reading a grower magazine last year about plant habit can have a lot to do with where the cuttings were taken on the tree. While no work was done on crapes specifically, they were being looked at being possible future test subjects.

Sassy,

As Stompede said, different crApe myrtle cultivars have different growth habits. Some tend to be more vertical, some spreading, some arching, etc.

Don't cut the tops off! That's called "crape murder" and will ruin the form of your crapes.

Rejuvenating pruning (removing about a third of the larger trunks per year) can be used to produce more trunks and a broader form. I personally think most crapes look best with somewhere between 3 and 7 trunks, but I've seen some impressive looking larger crapes with more trunks than that. The multiple trunked specimens tend to be wider and more arching than the ones with fewer trunks.