The amount of curl you get depends on the player that's taking the kick, the distance you’re taking it from and the power you put in. Players like Beckham and Recoba can get a lot of curl on their crosses and shots. I've actually scored quite a few goals from corners.
This is what I do if I want to score from a corner. Firstly, I move the player in towards the goal a lot more, then I hold down the L1 button, side button (in towards the goal) and the square button (I've changed it with the circle button) until the bar is filled about 3 quarters of the way and then I let go and some times they go in off the keeper, very rarely do they just go straight in. However, most of the times the Goalie save it, or the Defender defends it.
Note: Left Footed players should take corner kicks from the right and Right Footed players from the left.