I think billboards posted in really high traffic is very effective.
When I am sitting in bumper to bumper traffic in Chicago - barely moving and inch at a time - and I see a billboard advertising the trains, that really gets me thinking. It really gets me thinking when I sit there and a train swooshes by full of people going a LOT faster than traffic.
I think the same thing would work for bicycling. Have a billboard where people are typically stuck in traffic and can think, "oh, God, I would love to be bicycling right now in this beautiful weather. I would sure get their a lot faster too".