Being able to curl a ball like Beckham is not a skill that you are born with or acquire over night. Extra hours of practice and constant attempts are what it has taken for Beckham to become one of the world's best free kick executors. The most common reason for trying to make a ball bend or curve is to get the ball around an opponent and toward an intended target, such as trying to bend a ball around a wall or playing a through ball behind a defense and away from the goal-keeper.
A major factor in getting the ball to bend is to put spin on the ball with enough power as to make it shift. Your non-kicking foot plays a major role in the procedure, if you are going to bend the pass or shot using your right foot so that it goes to the left, you should plant you standing foot a couple of inches closer to the ball then when you would when taking a normal pass or shot.
By doing this your foot will make impact an inch or two from the center and away from the side of the ball closest to you. This will help create the spinning on the ball and if hit with enough force the spin will cause the ball to begin to bend.
The harder the ball is struck, the more the ball will spin. The more the ball spins, the more the ball will bend.
Thierry Henry of Arsenal FC & France is known as being one of the worlds best center forwards, he also takes a devastating free kick.
A harder and more complex way of making the ball bend is to have the ball bend over the opponent and down to the target. This is usually done to shoot over the wall and into the bottom corner away from the goal-keepers reach. Practice this method by planting your non kicking foot next to the ball but an inch or two slightly in front of what you would when palying a normal pass. You should intend to make contact with the ball slighty above the center. This should help create enough top spin to cause the ball to dip down shortly after being struck. Allowing the shot to clear the wall but not the goal.
When wanting to bend or curl the ball, it is important to hit the ball in the correct spot and with enough power to generate the spin required to make the ball move. Keep your eye on the ball through out the whole shot or pass and understand that technique and where on the ball your foot makes impact is more important than anything else.
Keep practicing, I am sure both David Beckham & Thierry Henry put in that little bit extra after regular practice to make their free-kicks that little bit extra special.