The ability to track the ball is a critical skill, especially when it comes to passing - in this case, the "brock string" exercise (see the video embedded below) can be useful; get a long piece of string with a few dots on it and you can study in a long/near/long/near fashion - which forces your eyes to transition between long-distance and close-up focal points, working on your eye muscles. The faster you go, the faster your tracking can improve. There are so may depth-perception practices available on YouTube. Here's someone utilizing the concept for baseball:
What we're really working towards is a combination of eye tracking and reaction time. Through repetition, we identified three levels of passing:
First, do you know where the ball is, so that you can effectively meet it? Next, are you setting your platform properly in order to receive the incoming ball? Are your angles going to help the ball end up in the proper spot? The third piece, which we tend to think about less, is the reaction time when the ball breaks in the last two feet...how fast can you adjust? Using something like a reaction ball lets you practice those split-second decisions. It's a cheap tool, but it's something kids can use to train a critical skill.
Set some targets - try, for instance, to collect ten bounces in a row, then move farther back and throw it again and throw it harder - the ball will go wild.
You can also do handball off the wall - even a tennis ball will work for less wear-and-tear on the wall. You maintain a low crouching position and you practice tracking the ball.
The idea is to beat the ball, right? Tennis players are often excellent at this, as their entire motive is to anticipate the eventual location of a small, batted ball. Utilizing these concepts can help players develop a sense of anticipation for the varied spots that a volleyball may end up.
Next up, we'll tackle setting - now, how are you supposed to work on that by yourself? We'll see!