First of all, make sure that the seatpost is the right diameter by inspecting the notch/groove in the seat tube under the clamp. Are the sides of the notch/groove parallel to each other, and not touching or badly bent in towards each other? This could be a sign of a improper size set post (measure the inside diameter of the seat post to determine the correct diametter for the seatpost).

How about adding a worm-screw type stainless steel hose clamp right at the top of the seat tube on the seatpost? That should keep the seatpost from sliding down.