How to retrain tongue/mouth muscles for foreign language pronunciation?

This advice will sound like a joke, but getting drunk in your target language goes a long way. I studied Spanish for years but didn't feel like I had it "in flow", and couldn't quite nail down feeling comfortable communicating. A blurry month of partying in Spain fixed that. Within that month, I noticed the largest improvements after nights I had gotten pretty wasted talking to people in Spanish at local bars. My theory is that when you drink, you stop speaking from your pre-frontal cortex. You are forced to make connections to the language from deeper parts of the brain. Same thing goes for experiencing strong emotions and situations in a target language. Of course, you have to have enough vocabulary and grammar to be able to communicate if you try, this practice just seems to make it "click" more.

I've used 2 techniques when learning Japanese. However, keep in mind that there are very few sounds in Japanese that aren't in English, so my experience has been relatively easy.

When I say 2 techniques, it's actually just one: shadowing. Essentially you take a recording of a piece of speech and memorise it. You then practice saying it at the same time as the recording. When you feel like you are able to say it, play the recording in headphones and record your voice. Then compare your recording to the original recording.

I used news casts to practice because it was easier to find audio clips that also had transcripts I could read -- however an audio book would be ideal if you can get your hands on it. I ran into a problem just having facility with the succession of sounds, timing and tones. My "second" technique was to do this with music. I memorised a variety of popular music and then went to karaoke 3 times a week :-) Probably this helped me the most.

I still struggle quite a bit with tones. Japanese does not have accents on words, but it has 2 tones -- either rising or falling. Getting the tones right is often the difference between being understood or not. At first I could not hear the tones but practicing example sentences that used homonyms with different tones helped me quite a bit.

As I said, I never really had to struggle with learning how to pronounce a sounds -- it was much more in selecting the correct sound, or refraining from using English dipthongs, etc. However, I've taught English to Japanese students who have real difficulty learning how to make certain sounds. I have found that being very specific about how the mouth moves, where you put your tongue, how to use your breath, etc makes all the difference. Probably the best reference are textbooks meant for people who were deaf from birth. After that it's just practice and shadowing is the best technique I know about.

Since you did not mention IPA above:

https://en.m.wikipedia.org/wiki/International_Phonetic_Alpha...

http://www.nativlang.com/linguistics/ipa-pronunciation-lesso...

Question 1: The key is usually long hours in a language lab where you hear words and phrases, repeat them and then listen to yourself. This, along with class, works well enough for most people.

Some people cannot, at first, hear certain sounds in a new language. Usually a language teacher makes them aware of these sounds. If that fails, one could hire a phonetician (like Henry Higgins in the movie "My Fair Lady"), a linguist skilled in the way one shapes ones vocal apparatus to make sounds in a particular language or languages. Some language textbooks provide descriptions of how to do this.

Question 2: A language lab.

https://en.wikipedia.org/wiki/My_Fair_Lady

"it seems possible for someone to learn a foreign language and eventually speak with minimal accent or no accent“

The jury is still out on that. See https://en.wikipedia.org/wiki/Critical_period#Linguistics.

Time will do it for some people, but not for everyone. For instance I come from the north-east of England and had a very strong accent which people outside of the area struggle to understand. I then lived in other parts of England for several years and the accent slowly went away. I now have a fairly standard English accent that everyone can understand.

I know other people from the area that have lived away from home for 20 years and still sound exactly how they used to!

Also my accent comes back when I speak to people from "home".

My Spanish also improved with alcohol and a few months travelling around south america, where people are far less picky about the language than they would be in Spain and not too many local people speak English.

Repetition.

It's really that simple.

(1) Get an understanding of how to make the sounds from native speakers.

(2) Make a list of all the words you know you don't have a good pronunciation of.

(3) Repeat the words to yourself regularly.

That's it.

Some words will come easier than others, and ones you have mastered can be removed from the list.

I find it's best to do them in small blocks. Five at a time, or whatever you can easily hold in memory.

You have to practice. And listen to native speakers making that sound.

You have to watch their mouth shape and tongue position.

In the end it comes down to practice with specific intent of improving your pronunciation. Hiring a voice coach would be the next step.