If you understand a few biomechanical concepts and how they relate to the game of volleyball, you can learn to pass extremely well (unless you don't have that eye-hand coordination going on, in which case you might want to try gardening). First of all, start with your arms apart (while waiting for the serve or attack or whatever) with your hands in front of you slightly pointed forward (at a 45 degree angle to the floor). It's hard enough to get to the ball so don't try moving with your hands clasped together (you'll look dumb anyway).
Second, look at your feet and we'll talk about pure biomechanics. Are your feet staggered or are they both underneath your center of gravity? If you do have them side by side, then ask yourself a simple question: If I were going to hit you with a 70mph ball in the arms and you wanted to control that ball and maybe not get knocked over, would you stand with your feet side by side or would you stagger them so as to absorb some of the kinetic energy from the ball and control it? Another question: Do you think that you might have to move forward or backward in order to pass the ball? Would you stand with your feet side by side in order to move quickly? It's important to look at the movement aspects of passing when we want to improve the skill. If your feet were side by side, your most efficient movement would be side to side, and if a ball hit you from one of your sides then you would handle it pretty well (too bad they're serving you from the front). If your feet were directly in front and behind each other, then your most efficient movements would be forward and backward, and if the ball was hit right at you then you would be in the optimal position to absorb the kinetic energy of the ball and control it.
What's the answer? It's a combination of the two; your feet should be a comfortable width apart and staggered so that forward, backward, and side to side movement can be maximized.
Now for the other parts of this skill.
SIT! Get your butt down. If your butt is stuck up in the air and you are bending over completely at the waist, then don't expect to break any records in the 5-yard sprint. Movement is very difficult (for a number of biomechanical reasons) when you "hang" and bend over at the waist. When your butt is down, your knees bend and your center of gravity lowers; hence, body movements are more controlled and more easily changed (reversed). There is one concept, however, that we can obtain from this bad habit of bending at the waist: arm movement.
The pendular or "elephant trunk" movement of the arms rotating with the entire shoulder girdle, side to side, is a movement that allows the platform of the passers arms to always face the setter or target (I hope it's the setter). In other words, the passer's shoulder girdle (including the arms) rotate on the frontal plane and allow the passer to accurately pass balls in a 6 foot or so (depending on the height of the player) radius of their starting position with only one step. Of course we must take into account that the passer may never take that step.
So, sometimes it isn't the most important thing to get to where the ball is going(especially when it's impossible), but to get your platform, which should always be facing the setter, to the ball's destination. Next, shrug those shoulders! The shoulder joint is a joint that obtains its stability from the muscles around it (this allows us to do so many different movements with it). Therefore, it's hard to keep a good "solid" platform with anything that is connected to the shoulder girdle--unless we shrug the shoulders, flexing the trapezius muscles, and putting the shoulder joint in a more stable position (your arm movements are definitely restricted while your shoulders are shrugged). Next, straighten your arm (flex your triceps) and make the platform even more stable. Now, pass the ball.
Here are some concepts that might help your passing as well: