The Volley is one of my favourite shots but it is off course difficult to advice someone from behind a PC-screen, good advice can only be given on court.
My experience is that footwork is an essential part when U play a volley. When (assuming the person is right-handed) playing a forhand-volley it is important to hit the ball exactly at the same moment that your left food will touch ground, if this is not the case and your left food is already on the ground, then your volley will lose a lot of speed. For the backhand-volley the right foot will have to touch ground at the same time that U hit the ball. Off course a firm grip is also very important.
For drive-volley's or Smashes it can help to point with your left-hand point-finger towards the ball when it is high in the air. This really helps to focuss.