My first "real" bike and I almost died when the bungy strap of a pannier snapped and the hook got caught in the spokes, winding the bungee cord around the freewheel. The panniers at that stage had holes worn in the corners from rubbing against the spokes.
Even when I had the panniers full, so they kept their shape, they would tend to slide back and rotate into the spokes.
So get a rack like those described above, and some sort of non-elastic retention system for the panniers.