I use extra credit to get students to do things that I want them to do. For example, I like discussion assignments (I teach online) posted by Wednesday, but our school has a standard requirement that discussions are due on Thursday. I give students 1 point of extra credit for posting by Wednesday and 2 points for posting by Tuesday. The maximum a student can get in a 12 week quarter is 24 points, which will not change their grades unless they are really teetering on the edge of the next grade. Most students will go for the extra point, though, and I think the earlier postings yield more discussion activity. I do not let students do extra credit at the end of the quarter to bring up a low grade. I don't like to see extra credit as a substitute for learning the required material in a course.