Have to disagree about polystyrene tiles; they will do bugger all. As said by others, mass is the key thing, and an airtight seal is important too. If there are any cracks in the wall, you could caulk these as any tiny gaps will allow sound to be transmitted. There is no easy fix, the cheapest would probably be a 2 inch stud wall infront of the existing wall packed with rockwool and finished with a couple of layers of plasterboard. This will work better still if you space this new wall a cm or so from the existing one, and minimise the contact points (screws) between the two walls.