I received this question during my workshops in Miami last week. It's hard when you have an apathetic student, but there are some strategies you can use. First, try to find out what is behind the mood. I had a student who seemed apathetic, but really wasn't getting enough sleep at night. One of my girls didn't want to "look too smart" in front of a boy she liked. So, see if you can get to the root of the problem.
Second, consider the two aspects of intrinsic motivation: value and success. Students are more motivated when they see value in what they are doing. Perhaps they need to see the relevance of the lesson, or the enjoyment of the activity that you are using, or they may see value in the relationship they have with you. Capitalize on the value.
Next, students are more motivated when they feel successful. Give them open-ended questions that have no wrong answer. Chunk assignments or tasks into small bites, so they can succeed at each step. Success builds upon success, and you can help your students feel successful!