I haven't been to SA, so can't give you any specifics, just the general biggies -
1. meds - do you need any shots before you go? Or any that you'll need while you're away?
2. docs - make copies of passport, tix, etc to leave with family, not a terrible idea to take along a copy in case your stuff doesn't end up in the same place you do...
3. $$$ - have some sort of extra backup resource just in case
4. Travel insurance - getting sick or injured on the road can be a very costly ordeal.
5. It's hard in the excitement/stress pre trip, but try to get some relaxing time in so that you start of the trip in the best possible state of mind/body.
Have a blast, and enjoy - and hopefully you'll get more specific practical tips here in the next day or two!