That's a good question, and I'd also be interested to hear from anyone who has it dialed. I've fiddled around w/several variations, trying get the set-up as simple and safe as possible.
I end up w/something that looks pretty similar to what Petzl pictures on their Chest Harness Manual . Basically, a locker through the belay loop, attached to a short sling which is attached to quicklink on chest harness, then to another locker which attaches the device. This keeps the device running higher up than what seems natural--we're used to being secured at the waist-- but it makes for a nice "top-rope" type catch.
Girth hitching to the belay loop seems like it would expose you to unnecessary fabric-to-fabric friction and wear, which would make me a little leary of going that route.
I run my backup prussik off a locker that's through the two harness hardpoints. Keeps it out of the way and also seems to "backup" the belay loop set up by using a different point of attachment.