This is meant as advice to immigrants to the US--and perhaps some of those who may have been born in the USA yet grew up elsewhere--but would like to stop answering the where are you from question every time they open their mouth.
I grew up in Slovakia. My father was a linguist, my mother an opera star, while my older brother loved to make fun of everything and everyone (including but not limited to accents). In that particular environment I learned how to imitate just about any accent in existence.
As a result I received a lot of compliments after leaving Slovakia. For example, during my years in Rome, several people said I spoke Italian better than the Italians. Why, thanks for the compliment. :)
Nevertheless, there was something about English, something mysterious, something untouchable, a best held secret, that made me sound almost like an American, but not 100%. So, I received strange comments. For example: When you sing, you have no accent, but when you talk you do. You need to work harder on it! Why, thanks for the compliment! :(
After I moved to the US, I took special courses, such as theatre speech at Pittsburgh Playhouse Theatre School. Upon graduation (well, end of the course), my teacher said I was pronouncing every single sound correctly. My phonemes were perfect. But there was something about saying a whole sentence, the cadence of the language.
Ah, yes, I noticed quite early that Americans sing their sentences. I mean, every language is sung (though it may not seem that way to native speakers, but they all do): Americans just sing their English differently from everyone else (including the British).
Well, I sure can sing. I have a very good musical ear. I could hear the difference. I just could not emulate it 100%. It sounded unnatural to me. I felt I was exaggerating, even mocking when I tried to talk like that. Most importantly, while I could essentially do it, I had to do it consciously. I had to multitask. I had to think about how I say it, which left me with fewer brain cycles to think about what I say. And I just did not want to sacrifice the clarity of thought for the clarity of accent.
I continued working on it. I taught myself Transatlantic. All my effort was good. I was improving. But I still did not sound 100% American (except when I visited Slovakia, my friends were complaining that I spoke Slovak with an American accent--Gosh!!!!!).
Finally, one day Nicolet Theatre decided to produce Anastasia, and I was cast as Petrovin. I was also asked to help everyone else to sound Russian. No problem, I can do the Russian accent very well.
But the saving grace, the mystery solved, came when the irecto gave us each a copy of a Russian accent tape and asked us to listen to it three times.
On that tape the instructor gave us a very quick tip on how to sound like a Russian. He said that while "we" resonate the speech in the middle of the mouth, the Russians resonate in the back of the mouth, close to the throat.
He asked us to put a finger at each cheek and notice how the sound resonates inbetween the two fingers.
That was enlightenment! I realized that in Slovak we resonate in the back of the mouth. Not as far back as in Russian, but certainly not in the middle.
I asked the director whether Americans really resonate in the middle of the mouth. He said, yes. I then complained that my speech teacher never mentioned that. The director's reply was that I was probably trained the right way. Hmmm...
At any rate, when I resonate in the middle of the mouth, I "sing" the language exactly the same way Americans do. I have no choice. It is impossible for me to talk the way that seems natural to me. Funny thing is, it does not matter what type of accent I use otherwise. I mean, I like the Transatlantic accent very much. I use it a lot. I also use the so called standard American I was taught at the theatre school. Sometimes, I like to talk with a Southern accent. Whichever accent I choose, if I resonate the sound in the middle of the mouth, I sound like an American. If I resonate in the back of the mouth, I sound like a foreigner.
This also explains why my friend told me, long time ago, that I had "no accent" (actually, he meant I sounded like an American) while singing: I learned in the singing lessons to resonate through the middle of the mouth, but I never did that while talking.
By the way, if you want to sound British, resonate in the front of the mouth. It works! | 
