With respect to Mike's knowledge on the matter, there is far more to it than what he states, or even implies. I'll suggest a good place to start, and that is www.w8ji.com
Tom has about 10 very good articles on the subject, and if anything is lacking it is a study of ground planes. I understand he is working on that one for later publishing. While some of the data is rather deep, it is nonetheless some of the best mobile antenna data on the net.
If you want a more practical view, visit my web site.