I am an American by birth and I grew up speaking only English. In school, I began to study Spanish, but after several years of study I discovered that spending as little as two weeks in a Spanish speaking country benefited me more than all my time in the classroom when it came to developing communication skills. Don't get me wrong, I'm in favor of classroom study, reading books, and so on, but I am convinced I have stumbled on some useful tips for learning another language.
When I moved to Morocco, I wanted to learn the local dialect, which is an offshoot of Arabic, but which is extremely different from the Arabic of the Middle East, of textbooks, or even that of international newspapers and television broadcasts. In fact, there is almost nothing written in the Moroccan dialect of Arabic. It is an oral dialect, and writing occurs in a different language, usually Modern Standard Arabic or French. This was a perfect opportunity to test my hypothesis on how one (specifically, me) might effectively learn a language and develop fluency as quickly as possible.
Here is what I chose to do that I found useful for me. Hopefully, others will benefit as well.
You have to immerse yourself in the language. Get rid of DVDs, internet movies, etc. for at least six months. Don't spend lots of time with other expat friends. Find the places that locals spend their time and go there. Watch the television shows they watch. Listen to local radio broadcasts. Hang out in groups consisting of all locals except you. Listen to the flow of the language. Try to understand what is going on. Get in over your head as often and as deeply as possible.
Don't try to translate. Not ever. Learn to ascribe meaning to specific sounds and patterns in your target language. Imitate them. Don't let yourself think, "This means X in my language." Instead, look at an object and call it the name that locals call it. Translation and communication are completely different skills. Try to figure out how people communicate meaning, including their body language, facial expressions, hand motions, and vocal tones.
Pick one new topic every day or two and learn it by interacting with as many people as possible in conversations on that topic. For example, ask a friend how to tell time. Then, spend a couple of hours walking around town without a watch on and ask as many people as you can what time it is. Listen to how people give you the answer, the variations in the responses, all the different possibilities. Put your watch back on and hope people ask you and try to answer them. The next day, ask a friend how to ask for and give directions to a place, then go out and ask 50 different people how to get to a place that is far, but well known to you. Repeat over and over for different topics.
Talk a lot. Notice when people understand and when they don't understand you. Adjust your speaking style, accent, rhythm and so on as necessary. Embrace the fact that you will make people laugh. Get comfortable with feeling foolish, it is part of the process. Kids do it when they learn their first language. You are the equivalent of a child in this process. Be one. Ask lots of questions. Tell stories at random.
Get people to tell you jokes. Don't worry that you don't get them, that you don't think they are funny. Jokes contain lots of slang as well as lots of culture. Both are good tools to have in your toolbox. Ask and learn all the vocabulary and grammar structures, but also look into the culture behind the humor. It will give you insight into proverbs, other jokes, and personal interactions among friends and family that other foreigners may never have.
Really, the bottom line is, get away from people who speak your native language, and spend as much time as possible with people who only speak your target language. Then, be observant and listen, mimic, and speak.
Learning a new language -- how to become fluent