In order to improve hitter-coverage I use a drill that incorporates the full offense and use the drill at least one or two days prior to each match. You may want to use the drill more often until the team perfects it.
I use three block-its and have team members hold them, and move to be in front of the set if necessary. Then I stand in the middle back of the court and toss balls for the setter who sets outside, back and middle. The setter can already be released or release on the slap of the ball before I toss.
The hitters must not hit the ball as hard as possible but tap the ball into the block-it. By tapping it into the block-it as opposed to hitting it hard you have a more realistic rebound. When the ball is dug by a cover player the setter sets again choosing her hitter. With good hitter coverage one ball can be in play for several sets. Sometimes players don’t know exactly where to cover from their position to the hitter, but the rule of thumb will come into play and that is to be as close to the hit and as low as possible.
This is intended to be a very fast paced drill. If a ball goes dead, hits the net, or goes out of play in anyway, bat the ball off the court sideways and have other players shag the ball because the next ball will always be coming very quickly. After every 8 or 10 tosses into the setter we rotate players from the back row to the front row in their offensive positions.
Rotate the libero and DS as appropriate. It is imperative for this to be a fast moving and unpredictable drill. I feel the block-it is a very important training aid and I started my career with one and found that having at least three is imperative. It is also fun for the coach to be very vocal and encouraging in the drill.