Enrolling in popular university classes can be kind of a crapshoot. If you end up on a waiting list, try showing up for the first couple of classes anyway to boost your chances of nabbing an open spot.
Many universities have a gap between the start of classes and the end of enrolment. This period is a great opportunity to sit in on classes while you wait for that semester's roster to finalise. This trick is in the same category as reducing your bills by asking. Whether or not it works depends heavily on the teacher of the class, the amount of open spots, and the luck of the draw. However, one course organiser on Reddit recommends taking the shot and showing up anyway so you aren't left behind if you get a spot:
If you don't attend the class from the beginning, you are missing important information about how the class is run, what it will be about, and most importantly, what is expected of you. Knowing what's expected of you is important for getting decent grades in a time-efficient manner.
It also happens occasionally that after students attend the first couple classes they decide they're not actually interested, it's too hard, too easy, or they just don't like the prof. Investing this little bit of time scoping out a class before you register can save you from wasting a semester/quarter/trimester on a class you're not into, or having a Drop/Withdrawn/F on your transcript.
Not only does it help you stay caught up, but it may increase your chances of getting into a class after other students drop. As another Reddit user points out, some professors may prefer to recognise the students that show initiative:
The only way to get into a full class is show up on the first day and show that you're actually interested. If you don't and just happen to sign up after the professor drops the no-shows, the professor will usually drop you as well and put in the student who managed to show up the first day. This was especially true in the smaller classes (20 or less).
This is very obviously not a guarantee. Some professors may be very strict about a first-come, first-serve policy, and some may kick you out of the class as soon as you show up. But it can't hurt to take your chances. The worst they can say is no.