Try equal parts vinegar and dish soap. Should work like a charm. There are "green" dish soaps that I think work pretty well. Of course, just the vinegar should get you most of the way, especially if you are cleaning every day.
@Mary Beth has some awesome all natural cleaner recipes on here and on her blog "Cupcakes and Crinoline"
Hi Nancy, you will find a bunch of great tips here: http://www.hometalk.com/search?filter=shower+cleaning&search=&zipcode=