Things I find to be helpful for attacking the high ball:
1) Do not hit while moving backwards (your momentum is going to take away all of your power because your body weight is moving away from the ball). Instead, move backwards to a desired position, get set, and try to move forward during the stroke (Sometimes you cannot). Even if you feel out of position when the ball arrives, you need to hit a high/low ball from a solid base (by getting set). If you have to hit on the run backwards, than you should hit a defensive lob to restart the point.
2) Vary your stroke for high balls - Take back the racquet high, swing level, and and finish high. Too many people swing from way low to high on high balls, opening the face of the racquet, causing weak, lob-like shot. Essentially, it is not right to swing like your are hitting a ball at normal height and instead, continue to swing, past the normal contact point to reach the high ball. A great thing about high balls is that you can hit the shot mostly flat/even slightly downwards (close the racquet face) to clear the net/hit into the court.