Three areas that all coaches would like their middle blockers to be better at are blocking, pressing and tracking the ball. In the video clip below Keylor Chan, head coach of Samford and former head coach at Northwestern, demonstrates a progression of drills that he utilizes to train his middle blockers.
Coach Chan shows you five drills that he uses. He begins with three drills that utilize a weighted ball that he makes his blockers press across the net to him. He then demonstrates a live ball blocking drill using a volleyball that requires the players to read and use the correct hand. He ends with a drill to teach middle blockers to track the ball and then accelerate to it.
The YouTube video below has sound, so please make sure that your sound is turned on and that you have access to the site (some schools block access to YouTube)