I'm originally from France, and I used three methods to improve my english: 1) Reading in english. back in high school, when I was still a beginner, my go-to books in english were the Harry Potter series: not because I have a particular fancy for the books, but because I'd already read them in French so I could guess the meaning of most words without a dictionary. I actually extended this method to Spanish and Latin, pretty successfully.

2) Watch movies. I started with french subtitles, then switched to english subtitles. I stayed with english subtitles for quite a while as I thought I wasn't ready to remove them. Then I spent 6 months abroad and did not have access to subtitles, so I was forced to watch movies in english without subtitles. I had to replay a lot of scenes on my first couple tries, but picked it up quickly.

3) The single most important thing I did to improve my english is .. to stop speaking french. That's right. I've been leaving in the US for about 2 years now, and I avoid french speaking people like the plague. And if for some reason I can't avoid it, then I speak english with them. No french. See, a lot of the foreigners I've seen who struggle with the language tend to stick with other foreigners from their country. That's the worst possible thing to do if you want to improve your english. I've seen people who've been in the US for 8 years and can still barely speak english: they don't need to since they almost never actually speak it in their community.

Now I don't know if that works for everyone. I've met a couple other people who've used a similar strategy, and obtained similar results, but that's still not a very large sample.