I use scotch magic tape. Leaves no residue. It does have an annoying tendency to tear in the wrong place, the substrate is pretty weak and brittle I think. But the cleanliness is most appreciated.
For quick scans of roll film I recommend the Digitaliza from Lomography.com. Cheap, very quick mounting, reasonably flat, easy to shim to optimal height.
But for final scans, the betterscanning solution is the best.