If there's one skill on the beam that frustrates intermediate gymnasts the most, it has to be the back walkover. How many times in practice have you gotten your hands on the beam, only to realize you're slightly off and end up having to bail out? Probably too many.
The first and most important thing to remember, whether you're just starting to learn this skill, or if you've been doing it for a while, is that your alignment with the beam has to be perfect. There are no ifs, ands, or buts about it.
Begin by standing on the beam, as if you were going to go for the back walkover. Take a moment to notice how your body is positioned. Are your feet stable on the beam, without an unnecessary turn-out? Are your hips square with the beam, and not twisting to one side? Are your shoulders straight and square with the beam? Are your arms directly above your head in alignment with the beam? If any one of these questions are answered, "No," most likely you'll be having trouble doing a straight back walkover.
The most common mistakes are in hip and shoulder alignment. Most gymnasts who start out with one foot in front tend to rotate their hips and/or shoulders to compensate. This does not work. It throws your alignment off, and it only makes it more difficult. Make sure you begin with your hips and shoulders square with the beam.
Also, a related error occurs when gymnasts arch back to put their hands on the beam. Again, they try to twist their bodies, thinking they'll get one hand on the beam more quickly, and therefore more safely. Well, the problem with this again is that it changes the natural direction that your body should move, and it makes your body loose. To correct this, be sure that you're going straight over the beam. Your shoulders and hips should remain square with the beam throughout the trick. Pretend there is a plane (a geometric plane, not an aircraft) running parallel along the length of the beam. Your whole body should stay in a constant relative position to this plane from beginning to end.
Another problem gymnasts have is that their back walkover is straight, but as soon as their feet land on the beam, they wobble and fall off. Why? Usually it's because they drop their chest and arms as they stand, which makes you loose. To avoid this, you want to lift your chest, keeping your arms by your ears as soon as your first foot is securely on the beam. Lifting your chest in this manner gives you less time to wobble.
And finally, as a last note, take the skill slowly; don't try to rush it. You will have more time to put your hands on the beam, and more time to get your feet where they belong.
Remember: loose body = wobble and fall. Stay tight.