Bandanna Headband and Ponytail
I have medium-length hair, and trust me, I've tried it all... My favorite and least time-consuming hair do is a low pony-tail (because a high one hurts under the helmet!) and a bandanna that's folded and tied around my head like a headband, under the ponytail. Not only does the bandanna keep my face clear of hair that is too short to stay in the ponytail holder blowing out from under the helmet while I'm riding, but when I take the helmet off the bandanna will kind of fluff up my bangs and camouflage the flatness.
Whatever you do, the important thing is to RIDE! :)