here's what I'd do. draw a line with a sharpie to indicate the post's height that you want
take it out
below the line, use your automatic center punch (or heck even a regular punch with a hammer) and made some "dents" in the seatpost in the range of about 1" below your marked insertion line
doing this will give the clamp some additional "bite".
the other thing you might try is a different seat post clamp. Somebody above mentioned a Phil Wood clamp. Your seatpost clamp puts a bending stress on the bolt as you tighten it. There are some clamps that have essentially a floating nut. These work very well ! The one pictured is $8 on ebay
I have the Stein knurling tool. This works very well too
/markp
