Yearly trimming of the tree

First of all, don't commit 'Crape Murder'....don't top the tree so that you prune off all branches at the top of the plant. You can't prevent the tree from growing tall, if you want to do that then don't plant a Crape. The best way to keep a Crape from growing to gangly is to prune off any suckers (small branches growing from the base of the tree) unless you are going for a multi-branched trunk....other than that, let the tree grow as tall as it wants.

Why are you asking how to look after a tree in florida on a european answers site?

It's Crepe Myrtle. Go by local plant nursery and ask their advice. Most are happy to help.