On Extra Credit
The first reason I don't give extra credit is because it's extra. That is, it's a reward for doing quantity of work. Most of the time, when I get a request from a student for this, it's because s/he is not doing so well and wants to raise the grade. Granting extra credit in such a circumstance is a disservice because the student is doing C work, let us say. Doing more C work should not improve the grade. If the student is capable of doing B work, then let him/her do so in the context of the requirements.
The second reason I don't give extra credit is because I've already designed the assignments I believe are appropriate to the course. New assignments wouldn't be so much extra as extraneous. There's a close relationship between the assignments and the basic design of the course; it's difficult to change the one without changing the other.
The third reason I don't give extra credit is because it's unfair. Every student, coming into the course, has made an assessment as to time required and how to budget their time to meet the requirements of the course. Most students are working at full capacity. It is fundamentally unfair that simply because this student has the time to do more, that student should be rewarded beyond what is available to another student.
There are other reasons, but this is perhaps enough. Extra credit has no place in my courses. It should be noted that "make-up work" is a synonym for extra credit.