Luck plays into it too. Enough people have to give up their spots in order for a spot to open up for you, when the class is full. But I assume that the class is not full as of yet, and from there it depends on how the school admits people. If they wait to fill the class before they accept waitlistees, then the above scenario comes into play. If the school admits students from the waitlist on a periodic basis, then those on the waitlist that are more competitive than others they interview will get the nod, so in this case it depends on other people.
Send them letters after you accomplish something new, like good fall grades or something. Send them a letter of intent or interest if the school is your top choice. It may not help very much to do this, but it certainly won't hurt. Good luck!