Tried this as an experiment and it seems to work very well. Paint a few coats of magnetic paint (that can be bought at Michaels or any craft store). This paint has metallic fillings in it that gives a surface for the magnet to attach to.
I was talking to a player and store owner about this method over gluing sheet metal to their bases. The advantage is that it can be painted over and isn't as obvious as the metal sheet. Sure you could primer over it and paint it but I don't think it would work as well.
The above picture has a long chain of flat magnets to show that it can hold weight upside down when challenged against Mr. Gravity. This will move slightly if you shake it so larger and heavier models may need a stronger pull that what I am using. For a single one in a base on say a plastic rank and line figure these guys won't go anywhere.