Admittedly, I'm hit and miss as to whether or not I have a second mag on me. That is something I'm trying to remedy, but I've yet to find a discrete way to carry one on my belt and not have it actually look like a magazine. Typically when I grab my mag, it goes in my front left pocket, since it is very close to my typical spot for mags, and I can pull it pretty fast.
I typically wear the 5.11 covert tactical pants, which have secondary pockets with pouches for mags, and I've used those too, but they're not the "quickest" for mag changes. I'd like a nice OWB leather style mag pouch that is easy to get into, but again, I don't want it to point out the actual purpose of the carrier.
I've also been dropping some pounds, so I have the pant room to make a hybrid IWB pouch that would help balance out the gun's weight on the other side, as well as be fairly comfortable. I've considered that option as well, but haven't gotten around to making one. It turns out mag carriers are harder to make than holsters. Truth be told, I wear my homemade MTAC/Crossbreed style hybrid holster pretty much all the time, gun in or not (i.e. at the office) just because it makes my pants fit better.