character development - How to make sure that you don't end up writing a Self-Insert?

Could I get some pointer as how to NOT write a Self-Insert/Mary Sue?

What I mean: according to Urban Dictionary a Self-Insert is "A story in which the reader inserts himself or herself; this usually occurs within a fanfiction"

Self-Inserted Characters commonly have almost NO personality (In order to asist the reader in inserting their personality onto the Self-Insert character, an example of Self-Inserts include: JRPG Protagonists that don't speak, Kirito,Red from Pokemon, etc.)

My Question: What are some general pointers that I could use in order to help prevent myself from writing a Self-Insert/Mary Sue?

P.S. Yes, it IS possible to unintentionally write Self-Inserts/Mary Sue's.