At the top of the pack, where the strap begins, there's an opening. I route the tube through that so that it follows the direction of the strap and hangs over the front of my shoulder. That way it's easy to grab with either hand to lift to my mouth. When I go down into the drops, I then attach the tube to the little clip on the strap to keep it out of the way.
Dunno if that's what most folks do, but it works for me.