Don't worry about amulets and powers and heterochromia, since those kinds of things don't necessarily make a mary sue. What's important to remember is that a mary sue is an unrealistically perfect character, and that perfection turns off the audience. That's why flaws are a good thing. We all have flaws, so the character will be more realistic. Remember context, too. If she has a good reason for having the powers, like it's normal in the world or something, then that's not a mary sue trait.
...all the guys/girls falling for him/her
6. Don't make your character win all the time (arguments, battles if it's action, fights, bribery).
7. Give your character weaknesses as well as strengths
8. Don't create a melodramatic girl/guy who thinks that they're nothing, while they're really pretty and everyone likes them (I know that's specific, but I see that often).
9. Make him/her get into trouble
10. Let him or her do stupid things sometimes
Hope I helped! :)
1. Do not give your character too many super powers (if it's fantasy, that is), or talents (in a non-fantasy story)
2. Do not make them stunningly beautiful (No purple-to-green-to-red-to-brown-to-blue-to-you-get-the-point color changing eyes, no perfect, glossy, sparkly, glittery hair, no flawless skin or face.)
3. Give your character three or more (the more the better) negative personality traits; expressed on a regular basis
4. Don't make every other character like them
5. Don't have...
Try to have them look to other characters for support and advice at times, instead of them coming up with the solution their problems themselves.
Have them make mistakes sometimes, and have other characters argue with them about their mistakes or flaws.
A quick and dirty way is to look up a list of character traits and choose flaws from there. Write journal entries from the point of view of the character about ways in which their flaws impacted their day, eg Mary is hot headed so she lashed out at Gary and now he's not speaking to her.
You could also study the people around you or online for their personalities. Get into what makes them tick, how they approach their days, flaws, etc. It takes time but it can't get more realistic than that.
Always list their flaws first. Of course, stay away from flaws like being weak, depressed, or not being able to do anything themselves. What helps me develop the character more is acting as if you were the character by adjusting your personality to theirs.