Is she living or is she dead? Living, with a story in progress.

Current Titans projects: Paved Paradise (Friends Like These is very close to being finished, and I'm waiting on Logan before working through the pre-set plot in Crystal.)

Current Tamora Pierce projects: Lady Lioness, an alternate universe where Alanna did go to the convent and Thom went to the palace. Another story with Briar and Tris may follow.

A Crash Course in Original Characters
So, your story has a bit part that can't be filled by a canon character. Instead of using a nameless security guard/scientist/police officer/victim/cashier #2 at the mall, you want to put in a character that will make a few small additions to your story. This is good. Developing Cashier #2 until she single-handedly saves the day with her spunky personality and unexpected genius-level IQ behind her size-00 (with curves!!) beauty is not good. Why? Because, no matter what you call Cashier #2, your readers will recognize her as Mary-Sue, that dreaded usurper of stories, third wheel on relationships, and general annoyance.

How do you avoid a Mary-Sue, and Marty-Stu, her just-as-attractive-but-in-a-masculine-way smart, rugged, athletic, and charming long-lost twin brother? Common sense, a little planning, and these easy tips.

1) Interaction
Your original character (OC) will have a reaction to the other characters in the story. That reaction should not (let me repeat: should not) be identical to your own. They might be impressed, cynical, distrusting, or fanatic. Whatever it is, that impression will change over time. An OC is not your cue to start waxing poetic about your favorite character(s). If your OC sounds like you, talks like you, and looks like you (but taller, with longer hair, martial arts know-how, and designer clothes), you don't have an OC. You have a self-insert Mary-Sue. Your readers will know, and will not share your opinions about Ms. Mary-Sue.

2) Role
Your OC is not there to save the day, play a pivotal role in the story that will earn the Most Valuable Character nod, or end up romantically involved with a canon character. If she or he is, then you aren't writing a story about (insert fandom here). You're writing a story about your character, and the canon characters seem to be participating. Your OC should be there to further the plot, provide something canon characters cannot, be more fun for the readers than Cashier #2, and get out of the way when the created role is over.

3) Career
You might need a scientist, police officer or Cashier #2. While your OC can certainly be qualified for the job, and very good at it, please respect the limits to suspension of disbelief. Suspension of disbelief is that old phenomenon that happens when you start watching a movie or a musical. If you go to see Wicked and keep wondering how everyone knows the steps and chords, you won't enjoy the show. If you refuse to listen to a donkey talking, you won't enjoy Shrek. Do you see how this works? Your readers do this for you, too, but you can't push them too far. You can have a microbiologist. You can even have a brilliant microbiologist who hardly puts two minutes into brushing her hair for lab, and then looks quite the sight when dressed for a night on the town. However: when you end up with Dr. Hensley, 22-year-old prodigy Ph.D. in microbiology, nationally renowned, runs her own lab, constantly solicited to appear on talkshows, brilliant, bombshell brunette with a whole lot of attitude, the most advanced research in the country, and a go-get-'em attitude- the audience is confused, because you just ended up with a very (very) bad case of Mary-Sue.

By doing your research, you will realize that earning your Ph.D. normally takes seven to nine years of very hard work beyond high school if all goes perfectly, especially in science. Doing it in four years total (bachelor's, master's, and doctor of philosophy degrees) means that you have a work-driven nutcase on your hands, in the best case. Owning a lab takes quite a bit of cash on its own, and the equipment required is enough to exhaust a bank. One tiny machine necessary for microbiology work, the faithful PCR machine, can be eight inches high and 8,000 dollars, easily. Last but not least- celebrity scientists are famous for something besides science. Typically, they have a crack team of experts to back them up, a brilliant orthodontist, a grade-A hairstylist, and connections to a talkshow host and/or a best-selling book.

Please, do your research. This applies to characters who are scientists, cops, zookeepers, firefighters, nurses, mail carriers, cartoonists, magazine editors, mechanics... You get the picture. If you want to add more to your story, know the background information.

That's it.
This is a crash course, written because profiles are boring when empty and I didn't have the right answer for "how do you write non-Sues?" before. If you want to test a character, I recommend a Mary-Sue litmus test for pure fun. Don't treat it as the gospel truth, but answering the questions honestly will give you a good idea where your characters stand. It's not always right, but it can give you a ballpark idea.

There are exceptions to the above rules, but I've never read a good OC that could break all three.

Mary-Sue Test

It's as easy as clicking boxes. That website even adds for you.
Note: link valid when the profile was updated, but searching "Mary Sue Test" in any search engine should find an alternative.

Updated November 26, 2011.
Status: Psychiatrist analyzed, psychiatrist approved. Actual writing set to resume soon, thanks for your patience.