How do you guys keep it from flailing about? Is there some way to secure it to the D ring or something?

strap on, eehhee.

secure it under the main strap.

Oops. I don't mean the underarm stabilizer strap. I mean the main strap. When it is fully strapped down I have a large piece that flies everywhere. I don't like to use the plastic thing on the strap to shorten it, because then I have to make it longer when I take the bag off. I was thinking a large carabiner to clip the end of the strap to the D-ring, but I don't know where to buy a large one.

Just tuck the loop up to the plastic bit through the d-ring. Holds it in place, and pops out easily when you need to tighten or loosen it.

Bring it up to the top so that it's more of a loop of a strap rather than one long strap, then tuck it over and under.

thats what i was saying. take the extra bit, loop it, tuck it under the mainstrap/itself.

use a rubberband

I use a carabiner. Use it to connect the loop at the end of the excess and the main strap. You don't need a large one. The most common size (what, about 2.5 inches long?) works perfect, and it's the kind most commonly handed out for free.

just tuck it under the strap that's going against your waist. that'll hold it, and you won't have to hassle with getting it out of a ring.

all done.

d-ring is annoying.

just get used to pulling the plastic (strap shortening) clip thing up and down every time you take the bag on and off

I use one of those "LIveStrong"bands, just slipped on the lower half of the strap to hold the slack close to the chest. this also doubles as a waterbottle holder FTW.

throw it in the garbage.