In the boxed method, the gown is folded in an acid free box. It is not sealed so you can take if out and admire it and the fabric can breathe. Your gown has less risk of permanent creasing, but you must make sure the box is acid free and not just acid coated.
Another option is bagging your gown. This is not your typical dry cleaner bag. It is similar to the method that museums use to maintain the condition of costumes. The gown will be protected from dust and light and will be easier to admire. You must make sure you use a reinforced hanger and twill tape for support to protect it from long term hanging.
Whatever method or company you choose to preserve your gown, make sure that you ask questions so that you understand the process. This is your gown from one of the most important days of your life so you want to preserve this memory.