Put a layer of electric tape around the post in the place you want to make a cut and cut it throughout the tape, the saw will have a grip ;)

I've done that before, I think with blue "painter's tape." I did it more to keep the ends from splintering. I've got some practice sawing bamboo (very similar properties to carbon fiber) and learned to tape it before you cut and you'll get much cleaner edges.

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com

Thanks for the suggestion. Shall I make the cut from the wide or narrow side of the seatpost?

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Paulo Matos

Wide. More surface area for the fine teeth to start a groove into. Gentle and light back and forth motion so it doesn't hop and skip.

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com

Watch out when finishing cut. It is very easy to split the last (surface) carbon fibers from the post in this case. It is not a big issue, always can be superglued :)
Having final few mm to cut, you can stop and finish it from opposite site (not necessary, an option).
Good luck

And do not breath that stuff coming from the hacksaw ;)

This ^^^ make sure to wear some kine of mask. You def son't want to be breathing it carbon dust