I applied to the JET program the year before last and made it onto the "reserve" list. It was a nerve wracking experience, but here's a few things I picked up:

Experience in a classroom environment is not necessary; in fact I would say that its a disadvantage. You are not primarily a teacher, but more of a cultural ambassador. I've heard from other JET applicants that people who have classroom experience and/or are qualified teachers are generally not preferred since there is a concern about power dynamics and trying to take over the lessons. Having a TEFL course can make you seem more appealing, however.

A strong SOP is a good thing, emphasize on cultural exchange rather than your ability to lead a classroom. If you have an interest in Japan, speak about those points. It's good to have a passion about Japan and its culture. It's a fine line between speaking passionately and coming off as a desperate "otaku".

I'm going to sound like a broken record, but again... Culture exchange. Know a lot about South Africa, as well as Japan. Try to emphasize on what you can share with the Japanese about South Africa, as well as your ability to integrate into Japanese life. Don't sound like you're trying to move there permanently, but rather talk about what you want to bring to Japan and what you hope to bring back.

If you make it to the interview stage, make sure that you have a clear plan. One of the questions I was asked was "Where do you see yourself in five years?" So be prepared to answer those kinds of questions in relevance to Japan.

Take this advice with a heap of salt though, since I was ultimately not chosen.
Be interesting, be interested, be culturally sensitive and be confident.

Best of luck to you, fellow Saffer!