If I'm carrying a semi-auto, the extra mag goes in the left front pocket. I've found that my wallet, which I keep in the same pocket, sits lower and keeps the mag upright with the bullets pointing forward. It works fine for me.
If I'm pocket carrying a j-frame, the speed strip with five more rounds goes between my body and the pocket holster, bullets pointed forward, in the right front pocket. In jeans, I'll typically use the key pocket.