Hi Lamb,
I've got a Camelbak Flashflo which is a waist mounted bladder which in my opinion is superb. It has external straps on top of the waist strap which you tighten as the liquid level lowers, it holds the liquid close to your body so that it can't slosh around, it really works well. It was reviewed in RW about a year ago and they rated it very highly as it is designed specifically for running.
I have attached a link which shows you what it is like, please note I didn't get it from this website, I told a family member that I wanted it for a birthday present and they bought it on line somewhere, don't know what site.
http://www.backcountrystore.com/camelbakflashflo.htm?CP=AltaVista