Layer it up inside with acid free tissue paper - john lewis or lakeland sell it. It preserves the fabric. My christening gown from the 1970's survived intact for dd. I change the tissue on it every couple of years.
I'd use an acid-free cardboard box too, or a inert plastic one (hard rather than flexible plastic is inert usually).
Make sure the air can circulate, don't put it in a tupperware-type box with a sealed lid or use those flexible plastic dry cleaner bags.
Crumple up the tissue loosely to fill out the sleeves etc so the creases don't permanently damage the fibres, and use tissue underneath as support. Unbleached calico is fine too.
Here's the firm I use at the museum for archival supplies http://store.preservationequipment.co.uk/store/store-entrance.cfm