I have chin-length hair that is quite curly and can go into overdrive after sweating under a helmet.

Since the hair on the top of my head is shorter I sometimes part in the middle and make 2 short braids and pin each with a barrette while letting the hair in the back do its own thing.

The easiest thing is to wear a Ficarre headband. These are metal but are the most comfortable headband I've ever worn, especially under a helmet. Plus they are very nice; I wear them riding and then again out to dinner at nice places (with my St. John suit for instance).

http://www.ficcare.com/Products.aspx?collId=36

So, when I'm hot and sweaty, I can pull off my helmet, pull off my headband, fluff my hear and be somewhat presentable.