Unmotivated 16 Year Old

My 16 year old daughter is very smart, but is very unmotivated to complete her school work, therefor seems to be running a C average in almost everything. I speak to her teachers who say she 'drifts off' in class. She often gets zeros for homework she 'does not feel like doing'. I realize a C average is not the end of the world, but I know she is capable of more and would like her to be able to go to college. I have taken away privliges, offered incentives, etc, and she says she will do better, and may for a very short time, then its back to the same old story. She is treated for depression. I don't know what else to do. I have considered cyber school, but she and my husband object. Any ideas?