I keep some in ammo cans, some in these aluminum cases I picked up at Harbor Freight years ago, some in plastic bins, all stacked on shelves in a pair of Ikea cabinets in the Gun Room. I need to work on pistol mag storage because they are all just tossed in to a couple tool bags next to the safes.
I do have one of those racks Ben linked to screwed to the wall next to my "bump in the night" AR but, while it looks very cool, I have come to question it's usefulness. What is the situation where I grab my AR and it's on board 30 rounds, sally forth to defend the castle, expend that mag, then fall back to the mag rack to reload and use the remaining 180 rounds that are staged and ready? It basically gives you a full basic load screwed to a wall for quick access. Unless you have a static fighting position, what's the use case for that? It looks baller though.