Your local bike shop should be able to sell you some carbon gripping compound, for example Finish Line Fiber Grip.
Other similar brands are available!
I've no idea what the seatpost clamp is on your bike
but I'm always suspicious of ones that are hand tightened.
I have 2 bikes with carbon seatposts & Ali frames, I use Salsa clamps that require an allen key & I don't experience any slippage.
bicycles.stackexchange.com/users/3412/Nick H
I have never used carbon frames or components, but I've had similar issues with aluminum posts in aluminum mountain bike frames.
In all cases the clamp was a hand tightened quick release type.
I also helped friends with this issue, some of them had carbon frames or seat posts.
Possible causes and correction applied where as follows: Clamp is not providing enough clamping force.
Adjust the clamp.
Almost always this kind of clamp has some quick release lever in one side, and a corresponding bolt or nut in the opposite side, tighten it a little bit at a time until problem disappears.
Seatpost or inside the seat tube is dirty or oily.
Debris, sand, mud, oil or even water between seat tube and seat post will create this kind of problem, also accelerating wear of both even when no apparent slippage occurs.
Clean both and make sure they are reasonably dry before putting them back together.
How does all the stuff get in there?
mainly the rear wheel trows it, it sticks to the seatpost and then, during cleanup it runs down into the seat tube.
Some frames are more prone than others to collect dust.
Shall this be the case, there are commercially available neoprene or rubber protections that seal the clamp area, but on the cheap you can use scraps from old inner tubes.
The quick release lever is producing too much friction: due to dirt and/or damage or wear, the quick release may be developing too much friction, giving the false feel of tightening, but the force you excerpt is being wasted.
Inspect the clamp and be sure it is not dirty or deformed, specially around the quick release pivot.
Friction surfaces of the quick release and their counterparts shall be clean, smooth and dent free.
Some clamps have a plastic, teflon or steel "cushion" that help these friction surfaces slide.
This must be well shaped and clean.
The friction surface of the quick release lever can be lubed with oil or light grease.
That also helps reducing the force required to securely tighten the seat post.
Dents and small deformations can be sanded down and polished with very fine grit sand paper.
There is dirt caught between the clamp and the neck of the seat tube: Dirt or other materials caught in there can also prevent clamp force to properly tighten the seat tube.
Clean it, the the inside of the clamp and remove any stickers or loose paint.
(Special attention with repainted frames here)
The inner surface of the clamp can also be lubed and it helps, but very little.
Clamp is misaligned: The seat tube has a cut part in the neck, this should be aligned with the clamp's gap (Unless some weird manufacturer says otherwise).
Check alignment, special attention if the clamp is an aftermarket one.
Wear: If your problem has been happening for a long time, chances are the seatpost or even the inner seat tube is worn out, thus having too much "play" between them and being too much flex for the seat tube to take up, so tightening it is not effective.
To be sure seat post and seat tube must be measured with a precision caliper.
Shall wear be the problem, hopefully changing your seat post is enough to solve the problem.
Final considerations:
Never use excessive force to tighten the seatpost (or any other component), specially with carbon components, you'll cause too much stress and reduce the time to failure of the component.
bicycles.stackexchange.com/users/2177/Jahaziel
If the carbon gripping stuff does not work.
Try a collar with two bolts.
These are typically used for mountain bikes.
Obviously you need to ensure it is the correct size for your frame.
I had the same problem mating my carbon cyclocross frame to my carbon seat-post.
Carbon gripping compound helped and seemed like the solution, but when it mattered (that is during the race) the seat-post slipped.
bicycles.stackexchange.com/users/2839/multidynamic
You can always go to a hardware store and get a stainless steel spiral screw hose clamp.
Wrap the post with a couple of wraps of rubber from an old inner tube, then apply the clamp, tight against the existing collar.
Tighten the heck out of it.
Always a risk you could crush a hollow post
, I suppose, but once you get that thing on the post isn't going to slip down.
bicycles.stackexchange.com/users/1584/Daniel R Hicks
I had this problem and looked into it at the time and the carbon gripping compound as mentioned ny Nick H, is the way to go.
But there was also a "cheap a&%e" solution of using either hairspray or the spray-on adhesive for sticking paper together.
You spray some on the seatpost and clamp it tight.
The light glue in the spray provides enough grip to keep the post in place.
But its not so sticky that it stops the seatpost from being adjusted later on (and it can be washed off)
bicycles.stackexchange.com/users/3392/nickdos
I have a Giant mountain bike with original seat and post.
It has been slipping side to side when I ride.
It is kept tight with a seat post clamp.
I was just able to get it tighter.
I needed to align the clamp with the opening of the seat tube (the receiving tube).
That is the part of the clamp which hold the screw or opening of the clamp must align with the crack or opening of the tube.
Of course you still need to adjust the clamp after.
bicycles.stackexchange.com/users/30682/stanhi
I bought a PVC tube of the same inside diameter as the outer diameter of my seat tube.
I positioned the saddle at the right height
and I measured the length between the seat and frame tube in which the seat tube engages.
I sawn PVC tube to this length.
I just have to engage the seat tube in the PVC tube and the frame tube .
The PVC tube prevents the seat tube to slip more than the length of the PVC pipe .This works for me now.
This works for me now.
bicycles.stackexchange.com/users/20991/yame2be
I got a  Cannondale Quick1 2010 , with the seatpost made with aluminium carbon-wrapped.  The problem is that the seatpost tends to slide downwards, even if I squeeze a lot the snap ring; if the ground is uneven the seatpost slips completely.  Is there a way to solve this problem?  Many thanks.
Problems with a slipping seatpost
bicycles.stackexchange.com
<seatpost>
bicycles.stackexchange.com/questions/8090
bicycles.stackexchange.com/users/3463/Sam Qasbah
There could be a number of reasons for a seat post slipping. There is dirt or oil on the post or tube. The clamp may be dirty or deformed. A carbon gripping product may solve the problem. Apparently, a cheaper product to do the same job is hairspray. Moreover, Salsa clamps or a collar with two bolts may hold the seat in place. Finally, the clamp alignment should be chacked as this may well be the cause. If there is dirt or oil in the seat tube or seatpost, there may not be enough friction to grip the post. Make sure the friction surfaces are clean and smooth, and remove small dents and deformations with very fine sandpaper. Carbon gripping compound, lube or the cheaper options of hairspray or spray-on adhesive are often advised, but may not be sufficient. A stronger clamp can help, but excessive force should never be used, particularly with carbon components. This problem can also be caused by misalignment of the clamp or a worn seatpost or seat tube.
If there is dirt or oil in the seat tube or seatpost, there may not be enough friction to grip the post. Make sure the friction surfaces are clean and smooth, and remove small dents and deformations with very fine sandpaper. Carbon gripping compound, lube or the cheaper options of hairspray or spray-on adhesive can help, but may not be sufficient. A stronger clamp can help, but excessive force should never be used, particularly with carbon components. This can be caused by misalignment of the clamp or a worn seatpost or seat tube.