We have had a number of brides who have sadly had to postpone their weddings for up to 12 months, which has meant their wedding gowns have to go into storage for up to 12 months. The information below is also helpful for brides wanting to keep their gowns in good condition after their wedding day as well.

We have put together a list of tips for our lovely brides for storing their wedding dresses long term in order to keep it fresh in the lead up to their new date.

Sarah Scaturro, the conservator in charge of the lab at the Costume Institute at the Metropolitan Museum of Art, said “Garments like to be where people are.” Therefore, a basement or attic is not ideal. Neither, she said, is next to an exterior wall where the temperature and humidity vary.

“Garments, like people, need a stable environment with nonfluctuating temperature and humidity in order to avoid damage,” she said. – Taken from an article in the NY Times. [2]

We have compiled these tips from a number of sources, please see the list of resources at the end.

There are certain fabrics, like silk and nylon, which tend to yellow a little as they get older. The most common cause of yellowing in wedding gowns is simply by leaving the dress in the plastic bag that the dress came with. Plastic gives off fumes that actually cause yellowing. But even with proper care, some fabrics will yellow more than others and it may be impossible to prevent all yellowing.
As a rule, silk fabric yellows more than synthetic fabrics, such as polyester, rayon and acetate. Nylon, which is a synthetic, also has a tendency to yellow more than other synthetic fabrics. Preserving your gown in an acid-free, climate-controlled environment is your best protection against yellowing. Padding your gown with acid-free tissue will help to prevent acid migration.
Keeping your gown in the best overall condition should be the main concern in preserving your bridal gown.
Remove it from the plastic bag and have it cleaned and preserved in an acid-free environment. [3]
Always use white cotton gloves when handling the wedding dress. [4]
Fold the wedding gown carefully, without creasing it. Wrap the sleeves under the bodice of the bridal gown, and then fold that back over the skirt, then wrap the skirt up and over the top of the bodice. After this, you can continue to fold the gown until it will neatly fit into the box snugly, but not too tightly. However, remember not to fold creases. The gown should be folded loosely. [1]

Use acid free tissue paper to place between each of the folds of the dress to help prevent creasing or the fabric touching other parts of the dress. Since many wedding gowns use special stitching or beads, sequins and pearls, you do not want these items to scratch or snag satin or silk fabrics when folded, and the tissue paper between folds will help prevent this. [1]
Once folded, wrap the wedding gown one more time completely with the unbleached muslin prior to inserting in the box for storage. This is a time-consuming process, so you don't want to package your dress until you are sure you will not need to remove it from the package again for some time. Wrapping and packaging your wedding dress over and over can damage the dress, so wait until you are truly ready to store and preserve your dress before you box it. [2]
To protect your gown from mildew and mould growth, it is advisable to keep your gown in a breathable environment. When fabrics can breathe, the humidity level remains constant around the garment as excess moisture dissipates into the air. The gown is at risk for mildew and mould growth if moisture can condense inside a box or any container. [3]
Once boxed, slide your wedding dress packaged box under your bed or in a drawer or on the shelf of a closet. The less light that can reach the dress while it is stored the better. [1]
When you are ready to reuse the gown or remove it from the box packaging, simply reverse the process and be careful not to snag or catch the dress or pull too roughly when taking it out of the box. [1]
Preservation should include protection from light, which means never put the gown in a box that has a see-through plastic window (the type dry cleaners often use). Other factors include protection from dust and insects; and the gown should never be wrapped in plastic. [2]
Boxing a gown is a safe approach if done correctly, Ms. Scaturro said. Boxes should be big enough to minimize the number of folds required in a garment, be archival and be designed for costumes, she said. “Use the largest box possible to fit in the designated storage space,” she said. Most cleaners use only one size container, but gowns come in many sizes, some with long trains. [2]
You can find some acid free wedding dress storage boxes here

Leading up to the new day, don’t try on your dress or take it out of its box until it is absolutely necessary. The more you touch it, the more likely it is to end up with oil and other stains that could get worse over time. I would say you need to check if it will need a professional dry clean before you get alterations. Some fabrics can shrink a little when dry cleaned – so if you hem it before dry cleaning, it may end up too short.
Make sure the wedding dress preservation box is sealed airtight - A preserved dress is typically stored in an airtight box where the oxygen has been sucked out and replaced with nitrogen. This prevents oxidation (aging, discoloration, etc.), which can happen to clothes that have been stored for several years. Wedding gown specialists recommend that you never break the seal, and if you do, that you should have it preserved and sealed back up again. [4]

References;
[1] https://www.wikihow.com/Preserve-and-Store-Your-Wedding-Gown
[2] https://www.nytimes.com/2013/04/14/fashion/weddings/proper-care-for-wedding-gowns- field-notes.html
[3] https://www.weddinggownclean.com.au/preservation-information
[4] https://www.brides.com/story/is-wedding-dress-preservation-worth-it

HOW TO STORE YOUR WEDDING DRESS