It depends
If I'm wearing a vest (not ballistic, just full of pockets, for cover) I always put a spare mag in the lower right pocket of the vest. This serves a two-fold purpose. First, I have a spare magazine, in case of malfunction or needing an extra fifteen rounds of ammo. Second, and more importantly, it provides a weight that keeps that side of the vest less susceptible to wind, and allows me to flip it open easily for my drawstroke.
If I'm not wearing my vest, it goes in my left front pocket with my cellphone.
I can hear you now: with his cellphone?
Well, it works very well. Since the magazine is longer than the cellphone, it's the first thing I hit when I stick my hand in that pocket. Furthermore, the cellphone wedges it in against the back of the pocket, preventing it from shifting around and not being in the precise location I want it to be.
It should be noted, however, that if I'm not wearing the vest, I often don't carry a spare magazine. It's probably not the safest move, but I already carry (literally) five or six pounds worth of crap in my pants. Adding in that extra half a pound of explosives and lead is just that much more uncomfortable.