Keep soap at its best through proper storage. Don't store soap in sealed plastic storage boxes or bags. Store soap out of direct sunlight. Keep your soap where it won't be exposed to extreme shifts in humidity and temperature.
Sealing lye soap in plastic will cause rancidity. Soap that gets lots of direct sunlight will heat up and cool down, causing rancidity through breakdown of oils, and scent and color will fade. Soap that is exposed to high humidity can get slimy because the glycerin in the soap is a humectant; it attracts moisture from the air.
If you make soap on a limited basis — 2 or 3 pounds at a time, for example — you can easily use a shelf on a bookcase for curing and storage. If you find yourself continually running out of room, get a set of plastic shelves you can put up and take down as needed. And if you get to the stage where you are a constant soapmaker, you may as well just give in and buy or make a soap-curing/storage rack.
Use soap in the order it was made. Soap that is older will last longer than a fresh bar of the same recipe. Be sure to keep “made on” and “use by” dates on your soap storage boxes and racks.
You can modify a bookcase or start from scratch to create your soap rack. The best ones have surfaces of wire mesh. Heavy wire mesh with ⅓-inch openings works great. Shelves that slide out for loading are ideal. Place freshly-cut soaps on the rack, spaced about ½ to 1 inch apart. Turn the soaps every few days to be sure they dry evenly. After a few weeks, when the surfaces are dry to the touch, you can stack them more closely. Try to space your soapmaking so that you can rotate partially cured soap out as you place the freshly cut soaps.
After a month of curing, cardboard or paperboard shoeboxes are useful for storing your soap. Line the box with plain paper — brown kraft paper is good. Place the soaps loosely in the box. Cut ventilation holes in the lid and sides so that the soap will be protected from light and dust and still get some air.
Be sure to always label your storage system with soap variety, date created, cure date, move-to-storage date, expected expiration date, and other information that will help you remember the details about each batch. Observe and record the changes your soap goes through as it ages. Again, keeping records is one of your most useful tools in progressing in your craft.