It appears you have an aluminum seat post. This is a common problem with carbon seat posts. In that case one solution is a special grease that has a granular substance in it to provide grip. I solved the problem on my bike by first torquing the seat post clamp to the recommended specs, and second placing a reflector with a clamp-on collar just above the seat post collar. The extra collar has been just enough assist to keep the post from creeping downward. A taillight with a clamp on collar would probably do the job, too.