Someplace dry, secure, fire resistant, and if we are talking more than a pound or two probably not in or really near the house.
I have heard of refrigerators being used, but those were non-functioning refrigerators.
Military Surplus ammo boxes seem to be popular, but they still need to go inside a fire resistant, rain resistant, somewhat secure place that probably does not want to be inside your house or garage.
If it damages your house, your insurance company will dis-own you before they pay.
There are federal regulations (The ATF site has links to regs, with the descriptions of the various types of magazines, distances from houses, etc.)
There are possibly state regs in your state that you have to be aware of as well.
Most local fire departments are dealing with people with a couple pounds for black power hunting, and may issue you a very limited permit if they need to in your particular state for storage in the home. Larger amounts they may prefer you not keep in the home.
I would not keep mine in my beer frige. To start with, it has room for exactly three kegs, and there is no extra space in it for powder. Also, I would not want to walk out back a coupla hundred feet for a refill when my mug went empty
