Remove the plastic cover on the side & thighten up the bolts under it.
Might as well go crazy with the 12mm & 14mm sockets while your at it & snug everything else up.
If that doesn't work, IIRC there is a nylon bushing on the pivot pin. That's on the inner side of the seat.
Tighten up the bolts first & see if that helps.
Someone once posted up a schematic for the seats, but can't remember who.
Just goes to show that the mind IS the first thing to go!