This is a question that I am often asked and the answer is simply PRACTICE!
There are of course many techniques for improving language and many different ways of learning. This may be in classes at school, adult education classes, self-study or the total immersion method.
Spend as much time as you can reading and listening to the language. If you are a beginner, start off with short news articles, working your way up to the longer ones as you become more confident. This will enable you to get a feel for the language and to build up your vocabulary.
The most effective way of learning is to spend time with speakers of the language. If your village has a local twinning association, join it. You will get to meet the people of the country and get to understand the culture and traditions.
Fluency is described here. I have studied languages since the age of 11 (French and German) and did A levels in these subjects. However, I didn't think myself truly fluent until I spent my year abroad, two years into my degree course. This is because I had no choice but to communicate.
Several years later I started an Open University course in German, and only felt fluent after doing a residential summer school in Jena, Germany. I strongly believe that it is vital to spend time in the country n order to become proficient.
Of course, becoming fluent in a language is the easy thing to do, it's keeping up your skills that is difficult.