My method is, "quantity over quality." The more I listened, read, and write in English, the more my English I improve. That's all.
But I have a big problem. I'm living in Japan, and I've been working at a Japanese domestic company, so basically everyone around me only uses Japanese. It isn't a good circumstance to learn English. Even in such a circumstance, making a habit of, and having fun with listening, reading, and writing in English are key factors of success.
On my way to the office, I listened to English Podcast, for example, English as a Second Language Podcast, Anderson Cooper 360, CNN Students News, NBC Nightly News, and TED. I don't make any effort at all. I listen to these podcasts just for fun. Recently I can't be interested in Japanese TV program at all, and I mainly watch CNN, BBC, Discovery Channel, and National Geographic Channel at home.
After I bought Kindle from Amazon.com, it became easier to buy books from the U.S. than from Japan. When I buy books from Amazon.com, I get them almost in a moment, but I have to wait for books to arrive from Amazon.co.jp for two or three days. And it's quite easy to look up a word in a dictionary on Kindle.
After the Great East Japan Earthquake, I've realized that Japanese press couldn't be trusted, so I always compare the articles of the same topic on Japanese press and the foreign press.
Now I've been keeping English diary on Lang-8, which is a social network website for language exchange, and the BlogSpot. In the office, I write only in Japanese, but on the Internet I mainly write in English. It's really much more fun for me to communicate with people all over the world than just in Japan.
Using English isn't the way of mastering English. I have many things in English to listen to, read, and write, so I just do so.