The Bible does not tell us how short our skirts can be, how tight we should wear our shirts, or how much skin is appropriate to display. Christians have drastically different standards on how we should dress, because the Bible doesn't give us specific instructions on clothing.
However, it does have a lot to say about modesty of the heart. So obviously, God is more concerned that we have a modest heart, since wearing a loose dress that covers you neck to toe, paired with a heart that is anything but modest, is not true modesty at all.
Most Christian girls have some sort of vague personal standard for modesty. I think it is important not to force that on others, but I think that girls definitely have a place to say something to friend who is not dressing modestly (especially if brothers or other guys have mentioned that they have noticed this girl is not dressing modestly).
But it is a very touchy subject, because we don't want to offend people, but we still want to protect our brothers in Christ.
I think the two most important things to think about when dressing, is firstly, the fact that if we dress a certain way, it will make it harder for guys, and we do complain about guys being jerks, and this is a way to help them not be jerks!
And secondly, that we should be saving everything for our future husband. No young man would like his future wife to be sharing all that with every guy that walks by.