The trick to using bobby pins is to remember that it needs to be anchored down. When I was still in the Army (waist length hair + training), I had to wear it in a bun and I could get it to stay in place with 3 bobby pins, even though I have very heavy hair. This isn't a challenge, of course, it is just to give an idea that it can be done.
I made a a pony tail, twirled the hair around itself until it started coiling and directed the coil so that it wrapped around the base of the pony tail. Then, I got a bobby pin and slipped it through the hair at the scalp, through some of the hair in the bun and then poked it between the elastic of the pony tail, which would also help keep it in place. Repeat around the bun, concentrating on the top part, not the bottom.
If, even so, it isn't staying up, try a braided bun by braiding the pony tail instead of twirling.