After math quizzes or tests, I put the answers up on the overhead projector. Each student is to write a brief explanation of any problem missed. This is well worth the time involved for several reasons: 1) If a student doesn't know why he missed the problem, it prompts a question. 2) If a student repeatedly writes the same explanation, it seems to have more of an effect than if I say the same thing. 3) When I look over the papers, it saves me a great deal of time in evaluating missed answers. 4) When the papers go home, it is immediately obvious to the parents the types of problems their students are having. I learned this tip when part of the National Writing Project. It seems so simple, but changed my math teaching so much.