When I got my bike it had jam nuts on a bolt with a split ring through the head. That seemed like a good idea, but it didn't work all that good. I decided to go with a bal detent pin, here is a picture, think (not too sure about posting pictures) They call it a detent pin.
www.prifast.co.uk That works great, I did have to drill the threads just a little, I got a 1/4 inch, it fits tight and the seat has not moved since I put them in.
Mike