We give each of our tandem passengers a folder with some goodies and a first jump certificate. Included in the goodies is a Skydive bumper sticker.
I always say: "...and here's a nice bumper sticker you can put on your car. Or if you dont want to put it on your car, put it on someone elses. Preferably over their vote for Kerry bumper sticker."
I would say about 90% think thats very funny. Informal but encouraging none the less.