I originally tried to have mine attach via the backpack's straps, too (like in the movie), but it was very unruly, and kep sliding all over the place. So, I now just use a small loop of nylon strap, with a snap. I'm sure one of those "cord keeper"-types of Velcro things would work great, too. The loop goes around the pouch's loop, then around the shoulder strap of the chest armor.
Backpack or not, mine is always right on my shoulder, always in the same place.