If it's a round seat post you can add some texture by rolling it on a flat surface while pushing down on a wood rasp to do the rolling. Target the area that will be under the seat clamp with a little bit more each way up and down.

Basically you're doing sort of a DIY knurled texture with the rasp, which has teeth & not fine cutting edges as would a file. May not need much to effect a more solid clamping, but you can always do more if a little isn't quite enough.