There are some very funny helmet stickers out there too, unfortunatley what I find amusing will get me in Gliders bad books, so here are some I remember although they're not verbatum but at least clean.
"If it wasn't for women and bikes I'd be wealthy"
"Is there life after death? Touch this bike and find out"
"Two wheels or two breasts, sooner or later you're gonna have trouble"
"Honk if you have never seen a loaded gun fired from a moving Harley"
"Keep Honking... I'm Reloading"
"How fast was I going officer...not fast enough I guess"
"Sorry did my loud pipes interrupt your phone call"