Classroom instruction always is a very artificial situation of linguistic communication in a foreign language. Motivation to communicate in a foreign language, on the other hand, normally comes in real-life situations when the need to communicate in a foreign language "naturally" arises. That understood may also be the approach with a solution in mind.
Practically, I'd suggest that you use more group dynamic elements in your teaching and give your students a lot of opportunity to use English in class by role playing and similar activities.

Another good way to motivate them would be to create multimedia-featured and interactive language quizzes for the web or a school intranet if available.

Good luck!