Goods in South Africa's pharmacies and grocery stores are very similar to those in the States. If you have a favorite brand of toiletry, bring it; otherwise expect to pay average prices for items you may have left at home. Minimarts at gas stations also stock the same range of candies and sodas you would expect of a roadside shop.
Take care to bring enough prescription medicines and a copy of your prescription if you anticipate needing a refill. Ask your doctor to write down the ingredients so that a pharmacist will find a suitable substitute if necessary. Obtain your antimalarials at home.
Incidents of theft from checked baggage in Cape Town and Johannesburg airports make luggage wrapping a popular option. For a few rand, the process of wrapping a bag in impenetrable cellophane is a great deterrent against crime. Fragile items in soft-sided bags can be crushed in the wrapping process, so remove breakable items or place them in the center of the bag to prevent their getting squeezed. The wrapping can be removed by hand or with a knife. Security accepts wrapped bags.
In southern Africa it's possible to experience muggy heat, bone-chilling cold, torrential thunderstorms, and scorching African sun all within a couple of days. The secret is to pack lightweight clothes that you can wear in layers, and at least one lightweight fleece pullover or sweater. Take along a warm jacket, too, especially if you're going to a private game lodge. It can get mighty cold sitting in an open Land Rover at night or on an early-morning game drive. It really and truly does get very cold in almost every part of southern Africa, so don't fall into the it's-Africa-so-it-must-always-be-hot trap.
South Africans tend to dress casually. Businessmen still wear suits, but dress standards have become less rigid and more interesting since the late Nelson Mandela redefined the concept of sartorial elegance with his Madiba shirts. You can go almost anywhere in neat, clean, casual clothes, but you can still get dolled up to go to the theater or opera.
It's easy to get fried in the strong African sun, especially in mile-high Johannesburg or windy Cape Town, where the air can feel deceptively cool. Pack plenty of sunscreen (SPF 30 or higher), sunglasses, and a hat. An umbrella comes in handy during those late-afternoon thunderstorms but is almost useless in Cape Town in the winter, as it will get blown inside out. But do take a raincoat.
Some hotels do supply washcloths; some don't. It's always a good idea to have at least a couple of tissues in your bag, and moist towelettes, because there may not be a restroom (or toilet paper) just when you need it. Even an hour in a safari vehicle on a dry day can cover you with dust.
Make copies of all your important documents. Leave one set in one bag, another at home, and try to save them online in a PDF file, which may be the fastest way to access data if you need to replace anything. Consider carrying a small card with emergency contact numbers on it, such as the local U.S. embassy, in case things go terribly wrong.
If your luggage does get lost, your best bet for replacing staple items will be Woolworths (a high-quality brand in South Africa, with both department and food stores). Edgars and Truworths are also fine. You can find all three in most big shopping malls. Mr. Price is South Africa's Target equivalent, but with less-reliable quality. Cape Union Mart is great for safari stuff. You'll find a wide supply of toiletries in Clicks.