I studied English at school from 8yrs old to 18, and I was really bad at it. After graduating high-school I decided to work on it (I wanted to work in IT, and english was a must-have). That was around the time when DVDs were really taking over, and it was now really easy to watch movies in original version (as opposed to the dubbed versions). So I started watching movies in English. Like a lot. I'm talking 3 to 5 movies a day every day for months. I took a year off between high school and college.

After that, I was fluent in listening, I moved to an english speaking country and started working on the speaking part. I was less intimidated since I could understand perfectly so I wasn't shy about speaking and exercising my pronunciation. I'm now fluent in English and live in the US. Most natives I talk to have no idea I'm a foreigner. Usually after an hour of conversation they start wondering which part of America I'm from and they're always surprised to hear I'm from Europe.

Starting at age 27 I learned French, Spanish, and Italian to a pretty good level: I can understand movies and usually communicate with native speakers without asking them to slow down or switch to English. I used a combination of Assimil (a book + cd series), reading news, listening to the radio, and taking some lessons on italki. I focused on French only for the first ~two years (at the time I was interested only in French and had no ambitions of being "multilingual" or anything). After that Spanish and Italian were much simpler to pick up thanks to shared vocabulary and grammar.

I had no background in linguistics or learning languages apart from (almost entirely forgotten) high school Spanish and a bit of classical Greek in college.

I learned 80% of my current (high) level of English when I was about 14 in about a year, besides the 3 or so hours at school I tried to immerse myself in the language any way I could, and I achieved a good reading, writing and listening level (spoken came with practice later), I did:

  - listen to songs and follow the lyrics  
  - read simplified books or children's books  
  - watch movies with subtitles in English  
  - listen to American radio  
  - read computer British magazines   

Picture dictionaries and regular English dictionaries helped too.

Extremely important, as in anything you want to learn is motivation, I was really motivated to learn.

My only "pro tip" is not to memorize vocabulary using isolated words, but complete sentences.

This was before the Internet, now I don't know what the excuse would be...

I moved to France back in 2011 with very basic French (read: virtually none). I couldn't order, I couldn't say anything well.

My technique: - I started with children's books with audio, but ensured I read for at least 1 hour every day - I did Anki religiously every day - I changed my smartphone, laptop, etc to French - I only tried to make French friends - I only listened to French music - I only listened to French (tech) podcasts - I bought a book on how to pronounce all the French sounds

I have to say that the first 3 months were very lonely. After that, I could talk, interview, and eventually joke in French.

I'm now fluent.

What could I have done better? I should have focused more on listening and speaking than reading. I love fantasy novels, and hence had a brilliant vocabulary on esoteric words (like different types of swords, etc) that were practically useless day to day.