I'm going to agree with attacking short balls. Early recognition and the ability to attack any short ball (slice, low, high etc.) will easily improve ones level. This also has the bonus of putting pressure on your opponent to hit deep/hard on every shot which can result in lots of errors.
In effect, short balls should be free points anyway. At the pro level a short ball is either a straight up winner or at, at a minimum, an easy follow up due to positional advantage.