No matter how good you get, hitting DOWN the MIDDLE and DEEP should remain a fundamental aspect of your strategy. A basic law in the jungle is that the advanced player will attack the net at every opportunity and the intermediate will wait for the first short ball before going in.
Anticipation is a key element in the development of an advanced player. An intermediate will often hit and back up or hesitate too long when getting his opponent in trouble or sensing when a ball will be short. The average player is average because he seldom hits the ball deep.
In fact against most players the odds are that the next ball or every other ball will be short so you should anticipate this probability and be inside the baseline expecting a short ball. Good tennis is a game of opportunities, it is a game of closing out a point and not letting your opponent keep playing. This means you must fight to get to the net.
The best shot to come in behind is a deep shot that keeps your opponent pinned behind the baseline. If you can develop the confidence to take the short ball and hit an approach shot and volley and come in, then you will find you anticipate always moving forward rather than naturally staying back. Your opponent will feel this pressure and this will affect his concentration and rhythm of his swing and he will produce errors.