I too keep two copies, as well as the originals - on different brands/manufactured discs if possible.
The originals - "everyday use" discs - are stored in a box, in plastic wallets, one set of back ups is stored in slim jewel cases on a shelf, in a cool dark room....and the other set of backups is stored at my mum's house, in wallets, in a box.
They all seem to be holding up very well, even the "everyday use" discs, since I switched to quality media (thanks to the guys here at CDF :flower: ).
Oh, and all discs are stored vertically...not on top of one another.
No idea if this is the best way, but as I say, it works very well for me!