A very common mistake many players will make when hitting volleys and half-volleys is to hit the ball too hard. A simple way to fix this is to think, "Should I add or should I subtract?"
 
When you receive a soft floating high volley, you should "add." That is, you should take the racket back and swing at the ball; attack and form the offensive position. Usually, you will receive these soft high balls and overheads about 20% of the time at the net. This 20%, you should "add," (swing at the ball). You should be thinking, "Winning the point."
 
However, 80% of the time at the net, you will receive low volleys and half-volleys. To control these shots you should subtract (return softer or slower). The first thing that too much power does on a low volley or half-volley is to either pop the ball up, which can be disarming , or you can hit it so hard it does not have time to lift over the net. Do not swing at these balls, rather block or punch the ball. Generally, you should be thinking "stay in the point" not "win the point." When you receive a nasty low ball try to return it with a nasty low shot to your opponent. Sometimes nasty and hard do not work together. Try nasty and soft. Wait for an unforced error or a ball that you can "add to."
 
Good luck on the courts!