Searched: "divided skirts for riding", "riding astride in a skirt" (which found me this article), "can you ride a horse in a skirt" (found me this Yahoo! Answers question) and similar searches.
I have a female character (aged 15-16ish, if it matters any). She's a fairly proficient rider, although I'm guessing she's somewhat out of practice (rode regularly until age 12-13ish, then lost the ability to do so due to lack of a horse to ride on, has been riding regularly again for a few months at this point in the story).
My question is -- I've gathered from my searches and articles that riding astride in skirts is possible (the Wife of Bath did so, for instance), but for the life of me I can't figure out exactly how it'd work. How exactly would she mount up in a skirt without showing everybody her legs or worse? Would she need help to mount up or would it be something she could do by herself? How would the skirt be arranged? Or would she be wearing a divided skirt (if those even existed)?
Also -- how much would riding sidesaddle rather than astride affect your control of the horse? (I know already that my character would hate it like hell, but the more I know about the differences, the better.)
I'm going to have her riding in trousers soon enough, but while she's stuck in skirts, I'd like to know what I'm talking about or at least sound like I do. :P
 Thanks to everybody who answered! I've got what I need now.