Having just come and gone at JNB 4 times as part of my April vacation here is my tip:

Do not let anyone "help" you with your luggage. Don't let them touch it. There are guys in vests who look like official airport workers allegedly "helping" people with their luggage, but they don't work for the airport and are not supposed to be there. Once they get a hold of your luggage they will hold it for ransom until you pay them a ridiculous amount as a tip.

I was actually screaming at them "Don't touch my luggage!" When they saw this 5'2" 60 year old American lady yelling at them they backed off pretty quick.