Keeping student interest is key in teaching a language, but the big question is the best way of doing that. Grammar-based instruction only captures the interest of about 4% of the students. That means 96% of language students don't enjoy grammar and they find language acquisition difficult because of the emphasis on grammar. Acquiring a language through reading and listening grabs the interests of more students, as long as they enjoy reading to begin with and are interested in the material they are reading.
Over all, though, I think the most effective way to inspire long term interest in language learning with an emphasis on proficiency (actual practical use of the language) is to combine this idea of reading and listening with an interactive teaching method called TPRS (Teaching Proficiency through Reading and Storytelling), a method that grew out of James Asher's and Stephen Krashen's studies. With TPRS, the teacher and students create a story together. The teacher has 2-3 specific structures they want to teach (like "likes to eat" or "has to go") and the students add details to those structures to make them into a story. As the teacher asks questions about those structures, it shows those structures in all their different ways of being used. For instance, the teacher could say, "What does Joey like to eat?" and the students could see how "likes to eat" functions in a question format. The students are more focused on building the story so they don't realize that they are gaining the correct format for structures as the story grows. Using so many questions enables the teacher to repeat the structure many times (Krashen suggests 50-100 times for a structure to be acquired).
After building the story together, the teacher gives the students reading material that uses those same structures. This builds proficiency because instead of students speaking out of memorized rules they speak out of what sounds right (which is what children do when they learn their original language). TPRS stories are created about the students so it engages their interest. It's an effective method because students are having so much fun that it doesn't seem like work. While they might not speak in completely grammatically correct sentences, the students of this method are very understandable. They also feel successful and that motivates them to acquire more of the language. Susan Gross, an excellent TPRS teacher/presenter, says, "Nothing motivates like success." The more successful students feel, the more motivated they are to do more with the language.