It needs to be made simple. Punch cards won't do it I'm thinking. Well maybe. You'd have to have all candidates listed twice. One set for first choice and another for second. If they pick the same candidate for both first and second, doesn't matter. In effect, they will have opted not have a second choice. Some actual testing of ballot designs with a representative sample of voters to insure the crap is understood needs to be done. Try out multiple designs until something is found with an obviously effective design.