I want to travel through Southern Africa for a month, what kind of clothing would be most appropriate?

From my experience working in Kenya, I would highly recommend learning about the community's cultural norms.  As an Asian girl, I stuck out like a sore thumb from the get-go, and I started forming better relationships with the locals when I started dressing the part.  That meant dressing conservatively - for ladies, absolutely no short shorts or mini skirts.  Everything at the knee or below.  And think twice about spaghetti straps and tank tops.  You don't have to agree with their cultural norms, but you're likely to have a better experience interacting with the locals if you dress the part.  As for men, the male students that I worked with wore shorts and t-shirts in casual travel, but say in Kenya, during work hours, you'll see many men dressed in dressier pants and shirts.  It all depends on what part you want to play. And consider buying clothes when you get there.  You're helping out their economy, and it's not expensive, relatively speaking.

If you are talking about travelling between August and October in southern africa, it is going to be warm to hot during the day and can get quite cold at night - so do pack so that you can dress in layers and have as much versatility as possible to deal with the temperature fluctuations (especially if visiting desert areas like the Kalahari or the Namib where there are the most temperature fluctuations between day and night)   Make sure you have some thin long sleeved shirts and pants - because the evenings might be warm but you do not want to leave limbs uncovered in shorts and tshirts because of malaria risk.   August and September should be dry season in southern Africa, October is shoulder season, so it might or might not rain :)

I traveled to South Africa this past December, and we just wore summer clothing. Shorts, tee shirts, and the like.