I was flipping through a copy of "Urban Bikers' Tricks & Tips" by Dave Glowacz at a friend's house the other day, and it had a bit to say about this; First of all, they suggested laying your bike flat on the sensor, as this will put more metal closer to the inductive loop. But, it also had a photo of this cool little product; a big magnet that you attach to the bottom of your pedal. This should trip even the most unruly sensors.