Hi Ali..The answer to your questions is this ....to speak with my accent ( American ) you would have to be born and raised here.  You will never lose your accent. And I can always hear an accent in a person from abroad no matter how well they speak.  Here , for example  is really how we speak : A guy says hey , whatcha gonna do when ya ain"t got no time? You gonna horse around until da ole lady comes home ?  I gotta go ta da game cuss I wanna see da game. I betcha we win !

I meet some Arabic students who spoken excellent English. I bet them that if I spoke with my friend in my Wisconin dialect they would not understand me. They did not believe it. So I spoke to my friend in the fashion we speak to each other . They did not understand what I said.

I met some bilingual Arabs who spoken American Engish perfectly but they were born into a home with immigrante Arab parents and grew up with American kids. 

Lots a luck...this all is just my opinion . I am sure some will disagree with me.

is not possible unless you are born and raised there. iisuperwomanii has GREAT english, but you can tell she is indian by her accent. she was born in india, and i think raised there. Many people can imitate a jamaican, british, austrailian, etc , accent. But I think no one can come close to imitating an american accent unless boston or southern accent.

Good points. Reasearch says the best age to learn a language is 5 to 8. A 5 or 6 years child can pick up native accent in less than six months if exposed to native environment in a native country. actually I was asking about the accent not "informal way" but, ur answer was really great. thx

i agree with ali, besides non english speaking countries like ours are not expected to speak like that as long as our english is neutral and understandable everything will be okay.