I looked at the post clamp system on that bike, and it does not look like a good design to me. A Wedge is inserted into the frame in front of the seat post. And held in place with an Allen head Bolt. You might be able to take a small piece of electrical tape inserted on to that curved edging the wedge just in front of the seat post. This might give you a little extra friction when you tighten up the wedge onto the seat post. Hope this helps.
