As with any trip there are a few things to consider such as type of travel, planned trekking, minimum allowable luggage on the plain, and how much space you need to leave for souvenirs.
Maybe stick to the old travel rule: “Pack what you bag, then, take out half the clothes and budget twice the money” 🙂
No matter the time of the year, if you are in South Africa for more than a week, pack for hot, cold and wet weather! The best advise is to research the area you are going and use http://www.weathersa.co.za/ to plan for the period.
When you are in the city or out at a camping site, there is normally a laundromat. Using the service will help limit luggage greatly on longer stays.
I recommend packing a day bag as well. Experienced trekkers know that hiking the mountains and urban trekking calls for different tools. So plan ahead. When in the city it is recommended to strap on your bags, make sure not to carry loose items and please don’t leave your bag unattended or behind you where you can’t feel or see it the whole time.
I personally travel with a medium size hiking bag for my camera gear with extra space for snacks and water. The bad allows for quick access and storage of my gear without taking the bag off my shoulders. A waste strap is especially useful on longer days to ensure the weight of the bag is taken off my shoulders.
My wife travels with smaller bag to carry the essentials like the keys, wallets, wipes, etc.
Think about the days you are planning and pack accordingly. It will make your stay so much more enjoyable. 🙂