The question of whether to use if I
was or if I were is a question of
mood, which, in grammarspeak, refers
to the way in which a verb expresses
an action or state of being. In the
English language, sentences can take
on three different moods:
Indicative: This is the most
common mood and the easiest to
understand. The indicative mood
makes statements or asks
questions: "My little brother is
bugging me." "Is that tuna
sandwich safe to eat?"
Imperative: An imperative
statement makes requests or gives
commands. The subject of an
imperative sentence is often an
understood "you" that does not
actually appear in the sentence:
"Don't bug your brother!" "Please
put that tuna sandwich in the
trash."
Subjunctive: A verb in the
subjunctive mood deals with
hypothetical situations or with
ideas that are contrary to fact. "If
my brother were a beetle, I would
step on him." "If that tuna
sandwich hadn't spoiled, I
would've had a nice lunch."
In the present subjunctive, were is
used for all people: "If I were a rich
man . . . " "If she were only ten years
younger . . . " "If they were only a bit
more experienced . . . " The past
subjunctive uses had in all cases: "If
my brother hadn't been bugging me,
I could have finished my homework."
"If I hadn't eaten that sandwich, I
wouldn't be in the hospital now."
Note that the words would and could
are good indicators of the subjunctive
mood, although their appearance
does not necessarily mean that a
sentence is in the subjunctive mood.
Now, to come around to the original
question, the subjunctive mood is the
most common mood in the if I was/
were dilemma, so if I were is more
often (though not always) the way to
go.
Not every if I statement should be in
the subjunctive mood. Consider the
following sentences:
If I was wrong, I apologize.
If I were wrong, I would apologize.
The first sentence is in the indicative
mood — it actually offers up the
speaker's apology. The second
sentence, in the subjunctive mood,
states either a) that an apology would
be forthcoming if the speaker's error
comes to light, or b) that the fact that
the speaker hasn't offered an apology
indicates that he or she was not
wrong. In either case, in this second
sentence, the speaker's error and
apology are both hypothetical, and
therefore the sentence is in the
subjunctive mood.

It depends on the subject

If I were you
If I was a doctor

You asked and answered.
Both are correct, depends on their usage.