Need advice on keeping seatpost to stay put

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I just puchased a thudbuster seat post for my Mountain Cycle-Rumble. Love the the seatpost but I can't seem to get it to stay put. The seat post is the correct size for the frame. After a few miles it seems to work it's way down about an inch or so. I've replaced my seat post clamp with a proper sized specialized quick realease, which seems to be clamping around the seatpost with plenty of pressure, but it still telescopes down when ridden. I'm beginning to get a little frustrated. The old carbon fiber seatpost with a shim never slipped on me so I am confused to why the new seat post simply won't stay put.

Thanks for any suggestions and you will receive much karma if yours solves my problem!

The usual fixes for this problem are:
1) remove excess grease from the seat post and seat tube.
2) tighten quick release seat post clamp, or replace with a bolt type clamp.
3) add a thin shim around the seat post, such as a piece cut from an aluminum can.