Rafael Nadal has won the French Open 13 times. He is the greatest clay court player of all time, if not the greatest player of all time. One of the keys to his success on all surfaces is his mentality.
He hits every shot with the idea that no matter how good it is, his opponent will make the next shot and he will be ready to do it again. This is an amazing mental approach considering that he is often hitting offensive shots that should have a good chance of winning the point. Since he approaches all of the points like this, he is rarely surprised when the opponent runs down a ball and hits a great shot in response to his great shot. This means that the opponent is getting very few free points.
In addition, when they hit a great shot, they have to repeatedly show that they are capable of putting several great shots together, several points in a row, throughout the match. This is an almost impossible thought process to execute successfully, and usually ends up becoming a string of errors or a lot of frustration. Nadal's persistence, relentless effort, and determination are all a huge part of his greatness.
Practice expecting every ball to come back by playing as if all of your opponent's shots are going over the net and all are going to land on the court. This will make your opponent feel that every point is a struggle, and that it is extremely difficult to win even a single point.