Here's my set up:

I have a small timbuk2 bag (i'm a pretty small guy as well) and I wear it quite low. This has always been the most comfortable position for me when riding or walking. With it low on my back, it is quite easy to swing it around and open it up very quickly. For support on the ride, I keep a carabiner (sp?) attached to one of the plastic loops on the bag, and then clip it onto my left-side belt buckle. It is easy to clip on and off, I don't notice it when I'm not using it, and it works perfectly to keep my bag situated exactly where I want it on my back. Just a bit of a different way of doing things than from what I've seen, you might want to give it a try.
PICS:

as it sits on my back normally:


flip it around to the front:


open 'er up and get whatever you need out, then flip back around


my securing system:




I can't think of a single thing I would change about this set-up, it works perfectly and is comfortable forever.