one thing that many people do not do. Make sure the ply is screwed to the sub flooring and make sure they are stainless screws or at least zinc coated. In a bathroom where water will be, reg screws will rust and pop very quickly.
Then too, insert a 2inch screw every 8 inches of space. This will stop all squeaks and will eliminate allot of forward problems.
When screwing down, make sure that the heads are counter sunk so they do not protrude up in the tile.