Erm, it comes down to alot of things but mainly just pressing square, try early crosses ( L1 + O ), this usally make the player volley the ball rather than head it, also you need a player with high tech, eg, Zidane, Maradona etc.....
Latest tech that i've been using is hold R2 when the ball is in flight and move the player away from the ball so it drops to his feet rather that his head, volley everytime...