You can really make decisive advances in your target language in as little as two weeks if you do it right , but you can also totally lose your time or money, which is alas the easiest and most common occurence. Go on the trip alone, so you won't be tempted to speak your monther tongue all the time. Live in a family with no other foreign student, so you will have to speak your target language for the most basic things, and you will hear it all day long. Group course are no good and you will need much more time.
Bad teachers do exist, and are well worth avoided.
Speak only in your target language , even with your con-nationals acquaintances Read the local newspapers , listen to the local radio, watch the national TV Try to find some books in your target language about some topic you love, and then read them slowly, writing down on flash cards the words you don't know. If you're a beginner, then limit yourself to the most useful words. Then sleep a night on the new flash cards, and review them all one time the next day. Try to find out the most common expressions and fashionable words . That will give charm to your speaking.