Preserving Toning

[Elle]

The Rhode Island School of Design Museum has a very small collection of ancient coins. Many years ago I visited the museum regularly to pay homage to these little gems, which were beautifully toned. Then one day someone polished the museum's collection of Paul Revere silverware, and as an afterthought went on to polish the Greek & Roman coins! They haven't been the same since.
Meanwhile I acquired a very small number of my own ancients. One, a common Roman Republic coin, was quite shiny-silver when I got it, but has taken on some really nice toning itself over the years.
I am wondering how to preserve its lovely state. I know enough not to polish it, but how do you store your collections of old coins in a way that will keep the air out & preserve the lustre? It doesn't fit in the usual plastic holders.
Thanks!