Because I feel the need to be pompous and declamatory.
-Do use original characters to fill gaps in the story if they’re needed. For example, you may need a DADA teacher in a Harry Potter story, and perhaps none of the canon characters fit your conception of what one should be like (or are prevented by something in the canon from assuming the post, such as Lupin’s werewolf problem). Calling your character “the new DADA professor” all the time would get annoying, so he/she will probably need a name.
-Don’t make your new character the center of the story, unless you want to risk both MS-dom and alienating a certain subset of your readers. Many fanfic readers, including myself, might tense up at the mention of a new character, but are probably willing to give the story a chance. I’ve seen two HP stories recently that had new DADA professors, both female and both pretty, but stayed good stories by focusing on the canon characters and being well-written. I’m part of the subset of fanfic readers that will flee madly when they see the new character becoming the center of the story, though, or the one with all the answers, especially if she has a mysterious past. If you’re willing to take the risk, then do become aware that you will lose some readers.
-Don’t whine about how people should give your powerful, beautiful, angsty OC a chance. You have to convince them from the way you write to give the character a chance, not just think they should out of the goodness of their hearts.
-Do write your character like a real person. Give her ordinary moments of frustration, impatience, unease, mischief, and laughter, as well as the extremes of terror, grief, despair, and love at first sight. This is a problem I see with a lot of OC’s; they could have been imported straight from a romance or gothic novel, where everyone believes in love at first sight and subterranean passages and black-and-white characterization, and everyone is dramatic (verging on melodramatic). Show me your OC laughing with the other characters over something someone said, or shutting her finger in a door and swearing about it. That helps build the foundation of sympathy for the character that telling me the long, sobby story of how her parents were both killed in a car accident doesn’t.
-Don’t give her an angsty past. Please. Almost all the OFC’s in Harry Potter are orphans, for example. Almost all the half-elven OFC’s in LOTR are outcast and scorned for their mixed blood (despite the canon fact that half-elves, aside from being very rare, have been revered as heroes: Elrond, Earendil, Elwing). Listening to the character moan about how her parents don’t understand her or how everyone hates her or how all the girls are jealous of her beauty doesn’t make me feel sympathy for your character; it makes me want to roast her over a slow fire.
-Do make her reasonably active- in moderation. A lot of Sues fall into one of these three types:
-The incredibly intelligent/intuitive/ass-kicking Sue who does everything because the canon characters are too dumb/dense/weak to do so.
-The spunky Sue who becomes key to the plot through an accident of birth, overhearing a conversation, or playing Nancy Drew through an incredibly unconvincing investigation.
-The passive, helpless Sue who angsts or moans all the time and lets things happen to her.
All of those are rarely people we encounter, either in real life or as full-fledged characters. Make your OC bold and clever enough to come up with some answers on her own, but not so powerful that she can take down the villains/solve the mystery/foil the forces of evil and save the day on her own. Let the canon characters take the lead, and fixate on something besides her once in a while. In a Forgotten Realms fanfic, perhaps your OC is the first one to discover that the drow are once again after Drizzt, but would she really fight all his enemies on her own, when Drizzt is one of the best swordsmen in the Realms and has extremely powerful allies besides? Probably not.
-Don’t endlessly describe your character’s beauty. Let us know what she looks like, if you must or if it’s going to be important to the story, but don’t keep harping on it. Compare the difference between:
-“Now I want you to pay attention,” Professor Morgana Fleur-de-lis said in her soft voice like the chiming of silver bells. All the students looked up at her, and the boys admired her flowing silver hair, her gentle cerulean eyes haunted with an old sorrow, and her movements so graceful they put unicorns to shame. Most of the girls were jealous.
“Now I want you to pay attention,” Professor Morgana Fleur-de-list scolded her students. She pushed her silver hair out of her blue eyes and scowled at them, pacing back and forth. “You, Weasley. Tell me how to defeat a Boggart.”
The name and the hair color would still make me say, “AARGH!” but I would keep on reading. In the first example, I would stop reading immediately.
-Do pick a point-of-view and stay there, unless you have clear breaks between viewpoint characters such as flashbacks or chapters. Nothing is more annoying- and this is also demonstrated in that first example I wrote above- than the author writing from her Sue’s viewpoint, except when she jumps outside to show how sorry other characters are for her, or how much they drool over her, or how jealous they are of her, or how the villains are plotting to destroy her. Unless Professor Fleur-de-lis is psychic, she won’t know what the students are thinking about her. She might guess, and be irritated, flattered, or indifferent, but it’s just sloppy writing to have a floating POV convey information that’s usually done with introspection and characterization.
-Don’t take canon characters OOC just so they can compliment or grovel or snipe at your Sue. Trying to shoehorn an OFC into a romantic relationship with Snape will turn most readers off if you portray him as really soft and gentle, just snapping at other people because they don’t understand him like Ashleigh Catherine Caitlin Amanda does. At no time in canon is he portrayed like this, and even if you argue he might be kind-hearted under the facade, it should take much more effort to break through that facade than just a few chapters of Ashleigh Catherine Caitlin Amanda batting her eyelashes at him.
-Do realize that not everyone will act like a teenager. If your OFC is an adult, don’t blame her problems on hormones, or give her misunderstanding parents, or, if she falls in love, make her act like a teenage girl with a crush. If she is a teenager, still try to avoid the cliches; they’re just silly by this point. Teenage protagonists are always always shoe-ins for MS’s, because the author, who is very probably a teenager herself or close to it in age, just can’t resist dumping her own complaints about her parents and school life and so on on the MS- in exaggerated form, of course.
I think that’s pompous and declamatory enough.