Seed receptacle candidates: paper envelope, small sealing plastic bag, old film canister
Seed embryos are delicate structures; when subjected to improper storage conditions they quickly lose viability. Light can damage and exacerbate deterioration if the seeds are left in direct sunlight. If the seeds become damp or are not completely dried when stored, they'll likely mold. And last but perhaps most important is to assure storage takes place in a cool environment. A basement, unheated garage, or perhaps even the vegetable drawer in the refrigerator are all possible locations where seed can be kept until it is ready to be planted.
What you store the seed in is also of some importance, though options are many. Old film canisters are ideal, especially if they are black (will not let any light through). With the advent of digital cameras these are increasingly hard to come by, but with some luck maybe found at some one-hour photo labs. Other options include paper envelopes and sealing plastic bags.
After an ideal container and location has been selected, be sure to label the seeds. Include the seed variety and the date collected.