In a formal situation," If I were she" is "correct."  In a normal conversation, 99% of the population would not use the subjunctive "were," and they would use the object form "her" instead of the "correct" "she" in the subject form. "She" is supposed to match up with "I" because "to be" acts as a linking verb, but most people ignore that rule here, or are at least blissfully unaware of it.

You would usually hear this, even though it is "incorrect:"

"If I was her, I wouldn't put up with that!"  

This is like how you would answer the question "Who is it?"  when you are knocking on someone's door.

"Correct" would be "It is I."  Almost no one would say that.  Nearly everyone would say "It's me." ( It is me.)

I hear tell that in French you say  "c'est moi" and it's considered incorrect if you don't use the accusative case. 

In the clauses you wrote, the pronouns are "object" pronouns = me, you, him/her/it, us, you, them.

The pronouns "I, you, he/she/it, we, you, they" are subject pronouns.

That's it...

I pretty much do what Phil does.

@Natalia

If you are writing something for school,  I would suggest that you use "If I were she."  If I were to write to a friend, I would write "If I were her."  However, I would be thinking about how I am breaking a grammar rule and I would be wondering if I should feel guilty. :)    

When I speak, I think that I adjust my English depending on who(m) I'm speaking with.

In college classes, unless it is some kind of creative writing class, you are expected to write formally.  In real life I would say something like "This chair I'm sitting on is really comfortable."  In a college paper that would be "This chair upon which I am sitting is very comfortable."

Many people would use "If I was  her" in conversation and informal writing.  To me, it just sounds "wrong," but to many people it would sound correct.  I would always say " If I were...."

@Kayla

So in everyday life, use "if I was her" is OK, but in formal situation, use "if I were she"? 

That would be fine.
I would normally say "If I were her" in a conversation.  "If I were she" sounds a little like you are trying to impress someone with your grammar knowledge. "If I was her" sounds wrong to me, but many, many people would say this.

Tempus' comments are correct.

"If I were she" is correct, but it is very formal and sounds odd, so most people will opt for the object pronoun "her".

Good explanation Tempus...

Thank you, Tempus and Phil!!!

I understand this sentence finally.