Nothing is really for sure when it comes to registering for classes.  My advice: make sure you plan which courses you have to take well in advance.  Make sure the math class you need for the next quarter is in your shopping cart on GET before the morning to register.  On the morning of your registration date, wake up at least at 5:40 at the latest to ensure your computer is working and you're all set to go.  Right when your clock says "6:00am" hit the submit button and hope for the best!  If that didn't work, the main thing to remember is "be persistent."  When crashing classes, don't walk away right away if the class looks full or even if the teacher says he might not be able to take everyone.  They say this to weed out the people who aren't motivated enough.  If you need the math class bad enough, you need to show the professor and they will eventually/hopefully let you in.  The cut off date to add classes is two weeks after the quarter has started and that give other not-so-serious students plenty of time to drop the class, and I guarantee you, they will.