When I was a kid decades ago, there was a series of books available from the doctor's office to explain any number of topics including menstruation. I haven't seen them in ages though.
Another resource we had then was that the sanitary napkin companies had a starter kit you could send away for that explained everything and provided a few supplies, too. Here's a link from Tampax about talking with your daughter (http://tampax.com/mothertodaughter1.php). Here's a link from the Always people (http://www.always.com/mom/).
In my school, it was a big deal in 5th grade that all of the girls were taken to the auditorium for a talk by the school nurse about "becoming a woman". The boys all went out to play kickball or something. It wasn't a sex talk per se, but she did show us slide of female reproductive anatomy and explain simply the menstrual cycle.
I also remember that my mother and I were invited with all the other girls to an evening session for a similar talk. I think it was given by the Modess people, a company that hasn't been around in years. That may have been through our Girl Scout Council though.
In my day, which wasn't that long ago, it was sanitary pads and belts. What a mess! It was like waddling around with a boat between your legs and you always had to be careful what you wore because you didn't want the outline of the sanitary belt to show through your clothes. Of course we couldn't wear pants to school in those days yet, but there was a lot of polyester skirts and dresses out there then.
The girls' network then sent out the word that The Diary of Anne Frank had a section in it where she talks about getting her period. My mom was my greatest resource though. She was very open and honest about talking with me and providing me what stuff was out there by following my natural curiosity. In those days, the presence of a big machine in the ladies' room that sold sanitary napkins eventually peaked any girl's curiosity to ask what was so special about those napkins and why they were in the bathroom.
My daughters both read the Judy Blume book in school. I guess that's how they learned. I remember them reading it and I've always wondered why they never came to me for that first talk. I have had many subsequent talks with them though. My younger daughter was also given a tape named something like "What every girl wants to know about sex" which she watched repeatedly. She used to also love to show it when she had sleep overs much to my chagrin.
You might want to check the public library and see what CD's or tapes they have available which might be appropriate. If you have friends with children the same age who are facing the same discussion, you might consider having a mother-daughter tea of something of the sort so that you can all watch a short movie and then chat about it. It doesn't need to be intense. Our menstrual cycle is a naturally occurring process, and it's very important for a young girl to know what to expect and what it is all about.
Even with all the preparation, the first day of my first period was a farce. It started at school and I had no supplies with me, so I spent the whole afternoon making trips to the bathroom to replace the wads of toilet paper I was using. Luckily, it was the afternoon I had home ec. Then when I got home to my supplies, I couldn't remember which one you could flush and which one (pad or tampon) you couldn't, so I took a chance and it was the wrong choice. I went to my parents' bathroom to get the plunger, and my dad came running in to fix the problem I knew how to fix. He saw what was happening and turned on his heel to get my mother. Mom swept me off in the car to go shopping and congratulated me on "being a woman". In the meantime, my two younger brothers were having a ball with my little incident.
My mom decided I should/could stay home from school the next day. She had classes, so I was home alone at age 12. I got adventurous and decided to try one of her tampons. It all went great until it was time to change it. I couldn't get it out and I was freaking about having to explain to my mom that I had a tampon stuck in me and couldn't get it out. I finally chilled out enough that my vaginal muscles relaxed and it came out, but I was in a panic for a while there.
Mom was good on theory, but there was some practical information I could have used back then.