A cliche is an expression that, when it was first used, was refreshing and different because it found a clever way to capture a well-known concept or experience in new words. However, since the expression was so creative at first, it caught on, and people used it so much that it lost its impact. Another problem with cliches is they have become so habitual to use, they don't take consideration to include them in writing -- they just slip in because they come to us so readily. For that reason, some of them are even frequently used incorrectly. As a result of all these issues with cliches, teachers discourage their students from using them because they want students' writing to reflect careful thought and creativity.
If you find you have used a cliche, the best method to remove it is to ask yourself, "What does this actually mean?" and replace the cliche with your answer. Here are some examples:
"the grass is always greener" could be changed to "people always think others are happier and more successful"
"it was a slam dunk" could be changed to "it was a skillful success"
"we put it on the back burner" could be changed to "we delayed implementing it"