I've been putting mine face up inside the Revelant gas tank pouch - this works great unless you have a wreck and can't move to your bike to hit an emergency help button. In that case it is much better to keep it like Toby mentioned on top of your backpack/hydration bag.
On the other hand if your bike gets stolen "as in the case with Mathew Lee on the Tour Divide last year" then you can track the thief to recover your bike