Definitely, open communication is the way to go! I knew several girls in high school whose parents put them on the Pill at 14 or 15 in order to feel they were safe without having to talk about it
: and I think that is a really poor approach. Combined w/lots of discussion of responsible sex, condoms, etc., it would be better.
My opinion of the Pill is quite negative, and especially I worry about girls taking it before their natural cycle has had a chance to get established. I would discourage a girl under 18 from taking it unless she either expects to be having sex within 6 months or has menstrual problems that haven't responded to other treatments.
When I started having sex at 16, my choice of contraception was spermicide capsules (the bullet-shaped things that melt in the vagina) combined w/condoms. My best friend advised me to tell guys we weren't going all the way unless HE provided the condom.
That was an effective way of making sure the responsibility was shared. Also, an advantage of the capsules was that I could insist on 10 minutes of foreplay because that's how long they take to melt!
I think barrier methods are the best choices for teens because they won't disrupt developing hormonal systems, and because the way they're used is linked directly to the sexual activity. With the Pill, Norplant, Depo, etc., it's easy to disassociate the use of contraception from the sex, which makes it easy to forget that if you stop using that you have to use something else. I think that being forced to pause in the midst of passion and make sure we were protected was good training!
A diaphragm or cervical cap is going to be difficult for a girl to insert before losing her virginity, or even after, unless she's very experienced w/menstrual cups or something similar. Also, she'd have to either wear it constantly just in case, or carry it around with her. An advantage of OTC contraceptives is that she can put a few in her purse, a few in her bedside table, etc., and always have one handy when needed.
If you do decide to put your daughter on a hormonal contraceptive, consider NuvaRing, which is supposed to act more locally (on the reproductive organs) than systemically and thus have fewer side effects.
Overall, I think the best thing to do is talk w/her about ALL the options available in contraception and their pros and cons, and let HER choose which method and when to start.