Over at How-to-box.com, Aaron Luhning offers a number of useful tips in his article "How to Shadowbox."
Although shadowboxing is a basic training technique, like any of the basics, I don't think a boxer can practice it too much or ever get so good at it that he doesn't need to practice it anymore.
While you're there, check out the rest of the How-to-box site. It contains many helpful pointers, videos, and discussions. Overall, it's a great site and I hope you'll find it beneficial.