The first drill is to try to hit only half volleys (just after the bounce) using the chopper/continental grip that we use for serving. Go for 10-20 half volleys in a row. Now work on regular volleys, again go for sets of 20-30 balls in a row. Again try alternating forehands and backhands at various distances from the wall. A great drill is to start far back and while volleying keep moving to the wall and see how close you can get! Its great for reactions and hand skills. You can even try hitting an approach shot then move towards the wall to play some volleys.