***** Before we start in on other Mary Sue traits, we're going to talk about cliches.
The Mary Sue test will interrogate you thoroughly:
Is your character "the chosen one"?
Does your character turn into a cool magical being?

A cliche is any idea that was once considered new and cool, but has been used so many times that it's considered stale and unoriginal now. "Stale and unoriginal" are the keywords - not everything that has been done over and over is a cliche. Some examples are:
Hero goes on a quest
The princess is rescued by a knight.
The hero fights to free his country or tribe.

These, and others like them, are classics. Many of them are so old that if they were once seen as "new", it probably doesn't matter anymore. Some writers dislike classic themes and think they're cliche, but for the most part, they aren't. Classics aren't original in themselves, but that's no problem. A classic theme only needs a good story behind it - and the story itself should be creative and original.
Cliches, however, are a different story.They're stale, dry and flat as paper, and, just as one piece of paper another, one writer's attempt at making a story out of a cliche will usually look exactly like another writer's attempt.
To be honest, with every "new" idea, whenever it is turned into a writing "trend", it's bound to become a cliche before long. If it's a good, well thought-out or exciting idea, the books or works that started the "trend" (or any good books that were written while the idea was still"hot") will usually outlast it. Those books, like Redwall, the Neverending Story, and the Giver by Lois Lowry, will remain popular and well-loved, even as the amateurs who try to imitate them fall flat.
You can't avoid every cliche - small cliches might creep into your writing at some point. Now when there are alot of small cliches in your story - more than 2 or 3 - or when your story's plot points, your character's personality traits, his/her relationships with others, or his/her goals - are based around cliches, you have a reason to worry.

There are adventure and personality cliches - these might be written well, but that would require a good bit of skill. After all, the success of these ideas lies in their originality, and they are no longer original.
Examples are:
The girl who discovers that her boyfriend is a "sparkly" vampire or werewolf.
A girl who fights, or becomes a warrior, to prove that she's as good as the men, or that she isn't a damsel in distress
A boy thinks he's an ordinary human, but discovers he's a member of a cool or exotic species.
Other cliches should be avoided because they're illogical, or simply outdated and out-of-place. For example:
A girl is a good person, but her parents are evil or devious (whether she's aware of it or not), and, besides being evil, they're also strict, bossy, overprotective, secretive, or treat her like a baby - traits that she hates.
A girl is good, but her parents are evil tyrants, and she spends alot of time arguing with them and thinking about how arrogant / narrowminded / irrational they are, or uses their arguments to prove that she's "not a child anymore" and has her own opinions.
(Talk about being self-absorbed!)
A good person is debating with a villain, who yells that the good person doesn't "know" him, or doesn't understand his past.

However, many cliches can actually be pulled out of the "cliche abyss." These are usually the simpler cliches (such as the "chosen one" theme), or cliches that deal with emotions, behavior and relationships. Examples are:
A girl is fighting against a group of people, but befriends one of them.
A girl and a boy are supposed to be fighting eachother, but fall in love.
The villain has an unhappy past that explains why he's evil.

The main thing that makes them cliches is their flatness. They could have been interesting or exciting, but weren't fleshed out enough, developed well enough,or spiced up enough. Or, the writer tries to base his story, plot, or character off of one of these cliches, which causes the story to become unoriginal. If you're going to try and retrieve a cliche, you should avoid making your story's plot, or your character's personality, revolve around it. The cliche should submit to the rules of your story and plot, not the other way around.
Let's take the villain we just mentioned. He's a greedy, devious character, who will kill in cold blood if it means he can get his hands on some money.Then, of a sudden, you throw in a tragic or unfair past - his parents' death, rape, an abusive childhood, a parent forcing hi to do evil things or teaching him to be a villain - to explain his actions. You've inserted a cliche into your story, and that cliche is now inhe reader views the villain.
But let's change that villain's past. What happened to him? His parents were clever merchants, who taught him the value of gold and silver. Then they were killed - maybe in the middle of a long-lasting feud, not at the hands of some random evil tyrant or ruffian. So our villain is a penniless orphan. What does he do? He takes over what remains of his parents' money, and starts a devious little trade of his own? What is this trade? You can decide for yourself - come up with a really original and exciting idea. What role does his feud with his rivals play? You can decide that too. You've taken a cliche and given it a full, fleshed-out (if still slightly cliched) story.
There are also small cliches - coming from a poor or ordinary background, but becoming a kid hero; having a cool sword; an elderly, wise mentor. These are great cliches as long as there aren't too many of them. They're pretty fun to read and write, when they have a good story behind them!
Throughout this essay, I'll give examples of cliches and how you can avoid or retrieve them. But now, let's move onto Angst.