I carry a spare mag in my back pocket. I admit that it's probably the stupidest place to carry a mag, because in a stick-up, they will go for your pockets and discover the mag (and then they know you have a pistol). This type of situation occurs if you hesitate to pull your pistol because you're not sure if you're about to be robbed. All it takes is a second or two of indecision and they're all over you. At this point, the gun in your waistband is useless.
I had the above scenario happen to me outside of an all-night deli. Thankfully, I wasn't carrying a spare mag that day (I had a 5-shot revolver with no reloads), and walked past some "gangstas". I contemplated pulling my pistol when my spider-senses told me to, but I hesitated. By that time, they were too close and had already started going through my pockets. They never discovered the pistol, but they did take $60 and my wallet from my pockets and let me go. I wanted to shoot them as soon as they turned their backs, but I decided that I was out of harms way and decided not to. Thank God I didn't, I would be in jail right now.
Someone really should think about making an IWB mag carrier instead of those bulky belt carriers.
ETA: Always trust your spider senses!