If you are trying to get into a class that is full, wait until the system purges students who have not paid their tuition and then get on RaiderNet immediately that morning and try to get in. You must keep track of the final payment of tuition dates to take advantage of this.
If it is a class that you must have and it is offered at no other time, consult the professor and the secretary of the department and get a POD (Permission of the Department) which is a waiver that allows you into a full class.
Sometimes it is a good idea to enroll in more classes than you actually plan to take. This gives you the laterality to drop a class on a whim and still have the number of hours per semester that you need. If you go to the first class and the professor reminds you of your Aunt Milly, whom you despise, you may choose to drop that class and still not go under your requirements. When you go to a class the first time and take a look at the syllabus and think to yourself, NO WAY am I doing all of this madness for only 3 credit hours, you can drop the class with impunity. It is a way to "audition"; a class and/or a professor.