It's always best to think ahead of time. Look at other courses you're planning on taking in the following semester that don't require any of the courses you're waiting for, and register in them now while you're waitlisted.
I'm at position 15 on the waitlist of a class that filled up slowly, so these people really aren't taking this class from an impulsive decision. With that said, this waitlist isn't moving. At all. So this morning I just preparing for the worst by registering in another math course
. Yeah, fuck math right but at least I have a fallback. I'm also waiting for IAT 103W to open up, and if it gets full before I'm able to register, I have HUM 102w to back me up
tl;dr: Have a back up course plan.