Is the seatpost twisting in the frame or is the seat itself twisting in the seatpost clamp?

A. If the seatpost is twisting in the frame, either you will have to tighten the clamp even more or your seatpost is the wrong diameter for the frame. Usually you will have problems with the height adjustment slipping also if this is the case.

B. If the seat is twisting on the seatpost there are two possible problems:

1. The seatpost's saddle clamp isn't tight. Just tighten it.

2. The seatpost clamp is a separate piece and is press fit into the seatpost itself (many lower cost seatposts are made this way). The press fit may not be tight enough so it allows the entire clamp and seat to pivot around the press-fit joint. The only fix for that is a replacement seatpost.