Hey I am in Volleyball too...
and in college when I practiced and scrimmaged with the club team...to keep up I incorporated Plyometrics to my workout.
DEFINITION : Plyometrics is a type of exercise training designed to produce fast, powerful movements, and improve the functions of the nervous system, generally for the purpose of improving performance in a specific sport. Plyometric movements, in which a muscle is loaded and then contracted in rapid sequence, use the strength, elasticity and innervation of muscle and as it was supposed to be surrounding tissues to jump higher, run faster, throw farther, or hit harder, depending on the desired training goal. Plyometrics is used to increase the speed or force of muscular contractions, often with the goal of increasing the height of a jump or the speed of a punch or throw.
Generally I stacked several large wooden boxes I would perform my 1-2-3 step volleyball spike and explode up onto the boxes.
It sounds easy but ITS NOT! after a 10 -15 reps you are thoroughly winded. It trained my muscles to really push my body up and increase my vertical jump.
also with the mirrors in the gym..you can see what you are doing wrong in your jumping technique
You can of course modify..add weights and so forth.
Look up Plyometrics (Google) you will find lots of suggestions.
Here is one site that shows you many different excercises: