Experts and BHG readers answer.
Thanks for writing. You can remove about a quarter or so of the oldest branches to help thin it out. Then if it still seems out of control, you can shorten some of the longest stems. I prefer a natural look, so I stagger the height at which I prune the stems, but if you wish, you can cut them at the same height for a more consistent look.
---Justin, Senior Garden Editor, BHG.com