I've had this problem before too. Here's how I solved the problem:
1. A rack where the rear struts are flush with the top edge of the rack (as shown in other posts). Here's what I'm using:
http://www.whymor.com/index.asp?Page...PROD&ProdID=70
2. I purchased Deuter Rack Pack II panniers from Sierra Trading post. Not the largest panniers out there, but the backs are super stiff, and they have a hook system that prevents pannier's from jumping off the rack. It amazes me how many panniers come with a super flexible back, which is the major culprit for panniers getting into the wheels (that and racks designed to allow the panniers to do that, like some Blackburn models I've owned).
You can also try a couple modifications to your panniers:
-re-locate the hooks toward the edges of the pannier--many hooks are too centered allowing the outsides to flop around.
-get some alumuminum bar stock from Home Despot and rivet or bolt it across the bottom/backside of each pannier.
Rich