I work in the foreign language education business, and so am naturally curious when I meet someone who speaks English without accent, but should have an accent. I have met 7 people that speak with no accent, but should have one, e.g. moved to the US from Taiwan at age 21, but is now 31 and speaks with no accent. They all told a similar story. They listened to the same piece of audio content thousands of times. For one it was an episode of Friends, for another guy it was a Bill Clinton speech.

If you want to make progress, I suggest finding a piece of content 15-30 minutes long, and listening and repeating everyday for a year. Once you have memorized the content word-for-word, then you will be able to stop hearing meaning, and actually hear the sounds.

You could also spend time with a book which explains differences like the 'th' difference in 'thin' and 'then'. I assure you that book will not help you. Many native english speakers don't distinguish between 'pin' and 'pen' but no one confuses 'thin' and 'then'. Trying to learn a bunch of rules will not work. You have to learn to hear the difference, and the only way to learn to hear the difference is to listen to the same thing over and over.

Absolutely fascinating method. I need to try this.

I have an actor friend, who is well-respected professionally. I once discussed how he approaches the new play that he is working on. He said that at first he just rote-learns the play, so that he can keep all of it in his head.Only after that he starts to analyze and think about the role, deeper themes and emotional flows in the play.

I also used a somewhat similar tactic to get better at public presentations. I studied and watched two presentations over and over, making notes about structure, pausing between words, content of the slides, etc. It really helped and I managed to create a presentation that people liked a lot and I was asked to give presentations afterwards in several events.

thin and then have a different phoneme at the start

I have succeeded with the academic approach, I can pronounce English sounds without an accent when I try and with a very slight accent when I don't try (most people can't hear minor mistakes when I speak)

I can tell you the difference between apical and laminal consonants and actually pronounce it. There is not just one way to learn. But I doubt most people can replicate my success without putting in as much time into it as I have. You have to be genuinely interested in linguistics

Pick an accent in English that sounds very extreme to you. Learn to speak that accent. For example, if you live in California learn to speak the difficult words as if you are from Texas or Georgia. Don't aspire to say "there" but go for "thar" instead. Example "can you hand me that book over thar?" Your goal should be to make people think, "this guy speaks good English. When he moved here he probably refined his English in Texas or somewhere in the South."

Once you have learned to mimic one accent you can quickly master another.

Source: I did this myself in Brazil when learning Portuguese. The words that were hard to pronounce I would say like I was from Rio. It was easier to mimic the accent from Rio because it was different from what I was hearing every day. If I were actually in Rio I would have probably found it easier to mimic a Paulistano accent.

Thanks for the answer, it's an interesting method! The main problem (I think) would be that self-study material tends to be heavily skewed toward either West Coast or East Coast accent, and I don't think I have seen anything on Southern accent.

I actually have been thinking about something similar for a while too. In my case, I thought that British accent would be easier for me to mimic. I haven't actually tried that out yet and now seems to be a good time to do so. Would British accent be a good goal? Out of curiosity, has there ever been a case of someone having a "too thick" British accent that would make it hard for American to understand?

Yes, you should mimic a British accent. Your first impression should be correct choice for you. There are different types of accents and some would be considered too strong to be understood, but you probably won't gravitate to those. I recommend you watch all episodes of Peep Show on Hulu. The characters are English with a few Scots. The shots are mostly close ups so you can really study the mouth movements. The characters are very affected and therefore easy to mimic. You should be able to say "right" just like Jeremy or "turkey fucker" like Johnson from the first go.

I find that I have to learn how the phoneme is actually said (where is the tongue, etc.) and then practice it a lot with recording(s) of native speakers (i.e. a recorded TV show) to compare with. Once you can make the right sound, practice it until you can say it easily.

If you have friendly native speakers, you may be able to get them to exaggerate the sound. E.G. the difference between thin and then is that the 'th' in then gets voiced (your throat vibrates) and the vowel is different. 'World' has an extra syllable (wur - ul - d) vs. 'word' (wur - d). I'm less sure about your issues with the others, save that certain sounds probably don't exist in your native language.

And those you just have to study & practice. I know how hard it is, though, having learned such sounds as 'ng', the 'eu' vowel in 'feu' (French - fire), glottal stops (yes, those exists in the English word bookcase, but...), the Japanese 'R'-group sounds (they're not 'R's or 'L's as we know them).

As others have mentioned, there are many different British accents. I wouldn't recommend a Cockney accent. However, a nice neutral British accent, like a BBC newscaster is a very good accent to have. Most Americans perceive people with such accents as being more intelligent and more attractive than people with neutral American accents. As someone who grew up in the Midwest and intentionally made my accent more neutral American while living on the East Coast, I have a pretty neutral American accent. However, now that I'm living in Hong Kong, I'm intentionally trying to pronounce my vowels in a slightly more British manner. However, I'm too attached to my Rs to switch to a non-rhotic British accent.

A slight Southern American accent sometimes does cause many Americans to perceive someone as more trustworthy, and some Americans do find a slight Southern accent to be very attractive. Though, be warned that a Southern accent may cause some Americans to perceive you as more stubborn and slightly less intelligent.

Britain has many incomprehensible accents, but as long as you stay in the realm of comprehensible ones you'll be fine.

Yes, there are plenty of American comedy skits about unintelligible British.

When I was learning to speak French, I struggled until my sister said "speak in a French accent".

I know, it seems trite. But the point is: think of someone who speaks your native language. Copy their thick accent into the language you are learning.

So when I began to use ze outrrrageous Franch acc-sont, my actual French-speaking improved.

My college German instructors told us to imitate Arnold Schwarzenegger's deep Austrian accent when we were speaking German in class. The idea worked for us too.

there are a few phonemes that are hard for me to discriminate but i've found asking friends who can discriminate to describe the physical arrangement of their lips/tongue for each of the sounds helps. i still can't discriminate between the sounds of certain phonemes but i can discriminate between how they feel when i vocalize them

Listen to Khan Academy videos (if you consider Sal's pronunciations as good as any American's).