What is a cliché? Cliché (pronounced klee-shay) is descended from a French word that referred to an early printing process allowing the printer to print multiple copies of a book or page without having to reset the type. In general, clichés in writing are expressions used over and over again to convey the exact same thought. You've heard these phrases before: someone was as busy as a bee or as mad as a hatter. Too many cooks in the kitchen spoil the broth, and it's raining cats and dogs, but you still look pretty as a picture. Put together like that, the clichés sound a little ridiculous. Spread through your writing, though, it's easy to forget they are there. It's okay to use some clichés--just try not to fill your writing with them. You can tell it's a cliché when your readers can predict exactly how a phrase will end (for example, "All's fair in love ________" or "be that as ________.")*
There are also such things as clichéd plot lines--these are fictions that follow a highly predictable plot. For example, before Book 5, almost everyone wrote Harry and Hermione as prefects--and were very surprised when Book 5 turned that cliché around. Can you still write a clichéd plot? Yes--but be aware that you have to be careful about that cliché.
For example, let's say you really want Harry to be an Auror. Is it necessary for Ron to become one too--as well as Hermione? Maybe Ron doesn't make the grades or Hermione chooses to pursue S.P.E.W. instead. Think about the plot line like it's a highway. Clichéd plots tend to be a straight line--the reader is taken directly from Point A to Point B. If you've ever driven on a highway for several hours, you know how this can rapidly become dull. It's more interesting if you pull over every once in awhile at a rest stop or get off the highway to visit a town. How does that translate into your plot? Insert complications. Harry doesn't get the OWLs he needs to be an Auror--or he does, and Snape still won't take him in class. How does Harry get around this roadblock? Does Dumbledore intervene, or does Harry give up? Ron gets more--or less--OWLs than Harry--how does this affect their friendship? If you plan to write a cliché, visit the plot section in the writing tips guide for additional help in making the plot strong enough to support your cliché.
Finally, some fan fiction authors tend to turn characters into clichés. This is discussed in more detail in the characterization guide, but in general, just remember that ALL people have more than one side. Voldemort is clearly evil, but he seems to be less cruel to some of his followers (Bellatrix as opposed to Wormtail, for example). Draco can be a little snotty, but he manages to have friends--so he's not mean to everybody. Remember--part of making a cliché work in your writing is taking it beyond what your readers expect. We expect Draco to snub Harry--but remember when they had to go into the Forbidden Forest together? The stereotype of Draco was temporarily suspended.
* "All's fair in love and war" and "Be that as it may"