It's all about positioning and timing. Most of the time your player will go for a header (if it's outside the area it's normally a weak header that will slowly roll into the penalty area).
In order for him to smack a volley on it, hold :R2: as the ball is dropping towards your player to control him, and move him slightly away from the ball and out of heading region. Then as it drops to about waist height (now the player won't be able to head it) hold :square: and the player should connect with some sort of volley.
It takes some practice so try that method on the training ground until you can pull it off effortlessly - trust me, it works. I hit a 50 yard pass to Socrates who ran past the defensive line and pinged a first time 21-yard volley onto the crossbar about 20 minutes ago. D'oh!