A good friend, whose first language is not English, recently asked me what was the difference between a cliche and an idiom. What a good question. After a lot of thought, I came to the conclusion that an idiom is a phrase whose meaning is not evident in its words. Kicking the bucket, buying the farm, peg out, pop one's clogs etc; all of them mean to die, but unless you knew that, you wouldn't know that.
A cliche, on the other hand, is a well-worn phrase that comes pre-packaged like a frozen meal. It has its uses, it's quick and convenient, but nobody is going to be bowled over by its style or originality. Over the moon, high as a kite, singing from the same hymn-sheet; these are all well-worn, familiar phrases. If you are writing to impress, leave them in the deep-freeze, and create something tasty out of fresh ingredients.