I have not tried this myself but according to Richard Masoner of cyclelicio.us all you have to do is turn your bike perpendicular to the flow of traffic in the rectangular box that is scored into the pavement at some stop lights.
Probably the most common vehicle detector used at traffic actuated traffic lights are inductive loops. These are coils of wires embedded into the pavement. You can often tell the location of these loops but rectangular cuts in the pavement at intersections.
These loops generate a magnetic field, and a chunk of metal traveling through this field induces eddies in the magnetic field, kind of like pitching a rock into a smoothly flowing stream induces eddies in the current. Big rocks, like big cars and trucks with big chunks of metal, induce big eddies. Small pebbles — like bicycles — induce barely detectable ripples.
Bikes are detected because the wheel (not the frame, as is commonly believed) generates these eddies. You can maximize these eddies by turning your bike horizontal. I demonstrate this at an intersection near my home. This works as well for my fancy carbon fiber road bike as well as the steel Schwinn I ride in this video.
Check out his video here:
and read the post here