I think you want to look for a "ring adjuster" - either a plastic or a metal gizmo that slips inside the ring. I have this kind of thing in some old rings and they work well
http://www.easycomforts.com/EasyComforts/Shopping/ProductDetail.aspx?CID=Jewelry+%26+Accessories&SCID=Jewelry+%26+Accessories&ProductID=311718