There are no best language acquisition methods. It depends on the learner. Each method has its advantages and disadvantages. Luckily, you can use more than one method to improve your language skills.
Watching DVDs can greatly improve your listening skills. It is best to start with DVDs for children, as they tend to be much easier to understand. Disney DVDs sometimes have over 15 language synchronizations with corresponding subtitles. Do not use subtitles in your native language but in the one you want to improve (that is, the same language as the synchronization). Otherwise, you will probably only concentrate on the subtitles without listening to the dialogs.
However, pay attention to the type of DVDs you watch. Some, especially sitcoms and the like, often use phrases that are not used in everyday speech. They use slang or accents from some part of the country that are not well understood in other parts. Thus, choose the DVDs carefully.
For those more interested in improving vocabulary and gaining a feeling for language, reading books might be one of the best language acquisition methods. Start with a book you really want to read, maybe even a good book you already know.
Be prepared that the first two or three chapters might be really difficult. Do not try and look up each word you do not know. You can guess many words from the context or because you already know another word from the same word family. Other words are not relevant to understand the sentence. Look up only the words which you cannot guess but that are important to understand the story.
The disadvantage is the constant interruption of the reading flow when you begin to read in a foreign language. You have to stick to it no matter how difficult it may seem in the beginning if you want to see results.
Language Courses in Foreign Countries
One of the most effective language acquisition methods is a language course in a foreign country. There is a vast number of companies that organize these language courses. The offers differ greatly in regard to duration, costs, accommodation (guest family, hostel, club hotel), intensity of courses, and number of students in each class. You should compare several offers before you make a decision. The best option for learning as much as possible (including cultural things) would be accommodation in a guest family, either alone or together with a second student.
However, language courses normally are very expensive so they are only an alternative for people with good income.
Tips for Teachers
You as a language teacher can integrate both watching DVDs and reading books in your classes. There are a huge number of books especially designed for language learners that have vocabulary help in the book (mostly at the bottom of each page) and only use a certain range of vocabulary. These books make it easier for students and are often about interesting topics depending on the age and skills of the language learners.
As for the DVDs, you can ask your students which ones they like. Let them suggest DVDs which are synchronized in the language you teach. You do not have to watch the complete film or DVD, just the interesting scenes. Give your students a questionnaire to fill in about the plot so you can check how much they have understood. Plus, students will pay more attention when they have something to fill in.