If your preferred section is full, please enroll in any section that is open that fits with your desired schedule. If that is not possible, you may need to consider adjusting your other classes to get into an open section.
If your conflict with open sections involves classes that you must take that semester for your major, then add yourself to the wait list for any other section (ideally the wait list with the fewest students on it). This will maximize your chances to get in as other students shuffle their schedules, or if (in some cases) enrollment caps might rise.
Make sure you have a back-up plan because putting your name on the wait list does not guarantee you will get any section of the course. Depending on the needs of your schedule, your back-up plan might be to take another section of the same course or a course in a different department. In either case, it should definitely be a class where the enrollment is such that you are certain to get an available seat.
Because you cannot assume that you will/will not get in to the section you are on a wait list for, you should attend BOTH the wait listed section and the back-up class.
Students in the Pratt School of Engineering should make sure to communicate any scheduling problems to their academic dean.
After the wait lists have been erased, if you were unable to enroll in any section of the appropriate course without sacrificing courses critical to your major, please contact the Supervisor of First-Year Instruction (SFI) at email@example.com. (See also "After Wait Lists Are Erased".)
Incoming students, you will not be allowed to join wait lists before you arrive at Duke in the fall of your freshman year; this is a University policy, not a Mathematics Department policy. You will be allowed to join wait lists in the week before classes begin after you have met with your pre-major advisor. NOTE: If you want to get onto a particular wait list as soon as possible, you can ask your advisor for their earliest possible meeting time, usually on Wednesday of that week. After your advisor talks to you about your courses and makes you eligible to enroll, you can then add yourself to a wait list.
If you are planning to join a wait list in August, then for now you might consider filling the fourth spot in your schedule with a course that you might realistically want to take -- perhaps an alternative elective, for example. This additional flexibility (you could drop the alternative elective OR the original elective) will make it easier for you in August when you are trying to get onto the best possible wait list at that time.