This is an excerpt from Mastering Skateboarding.
At the heart of skateboarding ability is balance. Missed tricks and falls are the result of losing your balance. Being born with good balance will help you progress as a skater. However, balance can be learned and improved with practice.
Balance is maintained by adjusting your center of gravity. Center of gravity is a term used to describe the place where the bulk of a person's or object's weight is concentrated.
When an object is at rest—be it a chair, alarm clock, or Labrador retriever—its center of gravity is spread across the width of its connection to the ground. This is why you have better balance when you stand with your feet apart than with your feet together. Think about a bowling ball: Its connection to the ground is a very small point, and so it's very easy to move the ball with a light touch even though it is heavy. It takes very little force to push the ball's center of gravity beyond its base. However, if you could find an object of identical mass and weight that was square, it wouldn't move with the same amount of pressure because its base is much wider—making it far more difficult to move its center of gravity beyond its base.
If you think about this principle in relation to skateboarding, it makes perfect sense. Any time the board contacts something that offers resistance or has changes in its pitch (such as a bank or a slope), your body needs to adjust. Hitting a rock, for example, can stop the board, but your body will continue moving forward. With a wide stance you have a better chance of adjusting in time and regaining your balance.
Center of gravity is also affected by its distance to the ground. When you're standing still, your center of gravity is right around your belly button. Bending your legs lowers your center of gravity and makes you more stable, whereas standing tall raises your center of gravity and makes you less stable.
When you are standing on a skateboard the same principles apply; standing tall with your feet together makes you unstable, while bending your knees with your feet apart improves your stability. When you are stable you can maintain control.
To see the benefits of a wide, low stance for yourself, stand with your feet touching each other and your knees locked, and swivel your hips as if you were using a Hula-Hoop. If you go crazy swiveling your hips, you may feel yourself losing your balance. Now try again with your feet about a foot apart and your knees bent. It is virtually impossible to throw yourself off balance with this stance. Keeping your center of gravity low and your stance wide is fundamental to good skateboarding.
You will find myriad balancing toys, mock skateboards, and devices that promise to improve your skating. The best way to improve your skateboarding balance is simply by riding a skateboard. This will prepare you for the small things that happen while skating and strengthen the muscle groups that skating requires.
Read more from Mastering Skateboarding by Per Welinder, Pete Whitley.