seat post slipping...

I have a 2010 Trek TTX 9.5.  I have to reset the level of my seatpost every time I go out on a ride because it ALWAYS slips down about 1.5 - 2 inches.  It doesn't matter if I ride for 45 min or 4 hours, by the time I get home, my post has dropped.  I have tightened it as tight as I can...well obviousy not as it's still slipping...at least as tight as I can get it...

Has anyone else had this problem?  How do I fix this?  It's a carbon, oblong seatpost.  When you tighten the screaw it pushes against the seatpost to hold it in place.  It's frustrating that I can't get it tight enough.  Can I shove anything down the down tube to prevent it from slipping?  This is so lame!

Grrr - any thoughts would be much appreciated!

Thanks