A pediatrician once told me that of all the signs of development it was hair under the arms that was the indicator of getting a period is the next step in development. I have seen it be true over and over again. Keep an eye on her arm pits and her moods and it will help you help her to get a better guesstimate of when she will likely start.
I would invest into a few good what's happening to me books. I highly recommend the American Girl Library (same company but a different series from The American Girl Dolls Books). Every girl should own a copy of The Care and Keeping of You. (Maybe they should give it out at the hospitals when mom's give birth to daughters and tell them, hold on to this book, you'll need it sooner than you think.)
http://www.amazon.com/Care-Keeping-American-Girl-Library/...
There are some really great books that have been mentioned in past posts, do a search for any of these buzz words: Care and Keeping of You American Girl Body Books what's happen period, etc. The archives are such a great resource.
Having books of her own in her room (even if she keeps them away under her bed or in a draw) is a great way to teach her how to locate information and take care of her self. Knowledge is Power.
I agree with having a prepared gift bag or special box with pads, small tampons, panty liners along with a small 10 or 20 year calendar (just number dates) so she can learn the habit of keeping track. Also a special women's only gift, a piece or art, a special girl's day out, something to mark the occasion as a life cycle moment. Also, have you seen New Moon Magazine for Girls? It is an excellent empowerment magazine which sounds like a great match for your daughter. http://www.newmoon.com/
Enjoy each chapter - Today is a Gift, that's why it is called The Present!