A cliché can be two things:

1. An overused expression, something that is said a lot that has become some common, it no longer really has any relevance or is even noticed in conversation. Phrases such as "to this day" or "next thing I knew" are examples of such a cliches, and you often say these phrases without noticing you are doing so.
2. An idea with a different meaning from its literal meaning. For example, the phrases "sweaty palms�" or "twinkling eyes" have come to mean more than the fact that your palms are just sweaty or that your eye's have a twinkle. When you say someone has sweaty palms, everyone knows you mean "he is nervous" because the expression has become a cliche.

There is a link in the discussion for a list to look at for suggestions:
http://clichesite.com/alpha_list.asp?which=lett+1