How do you not make your character Mary Sue?

Saying that a Mary Sue is a character that suceeds at everything, or everyone falls in love with, though this is the very basic form of Mary Sue. While making sure that ones characters has strenghts and weaknesses, one can't go and base that as the only thing that makes the characters Sues. This is more for fanfiction, then original work...

1.) Don't make characters OoC... When a canon character no longer acts like a canon character, they are an OC (original character) desquised to look like a canon character.

2.) Don't make changes to canon characters... For example, some fanfics I see, will go and change only one characters age and physical attributes, just to fit their OC. This is a huge no-no!

3.) Pay attention to canon... When you break canon, and make it fit your OC, instead of making the OC, fit the canon, it is rather taking liberties with the work.

4.) Be believable... Goes for both fanfiction and original work. Do ones research, don't make the reader come in and go, that can't happen...