Don't freak out

This is a rule you should continue to follow no matter what you do in life, but is especially helpful in this situation.

Email the professor

Around this time, professors are getting flooded with requests from students wanting to get into full classes. This doesn't mean you shouldn't burden them with your email; it means they are expecting interested students to email them. Send a short, concise message telling them that you are interested in the class and ask if there would be any chance for you to get in.

Attend the first class

Often, the advice professors will give you when they reply to your email is to attend the first class. The first class isn't the most important class in terms of what will be taught. However, attending the first class means you are serious about taking the course and aren't going to give up on it.

Keep attending class

Every student is in the same position as you are. They registered for more classes than they want to take and are "shopping." For the first couple of weeks, you can drop or add classes as you please, which means that classes that were once full will have spaces. If you keep attending class and keep up with assignments, odds are that you will have priority. Professors give preference to people who need the class for a major and then from higher to lower class year (senior to freshman).

Have a backup plan

For two weeks, or until I find out whether I get into my waitlisted class, I will be attending more than the usual number of classes. This is so that if I don't get into my waitlisted class, I won't have a credit shortage and I won't have to fall back in my backup class. Chances are that enough people will drop the class, especially if it is very difficult like computer science, and you will have a chance. In popular classes like art and psychology, odds are you probably won't get in, so prepare for that.

Remember that everything works out at the end

Life is full of surprises. So what if you didn't get into the class you wanted? Your life obviously has something else in store for you. It's your job to make sure you make the best out of what you have.