Place the item in an ESD bag along with a couple desiccant bags. Replace the air in the bag with dry nitrogen and then heat seal it. Place into another bag as b

Place the item in an ESD bag along with a couple desiccant bags. Replace the air in the bag with dry nitrogen and then heat seal it. Place into another bag as backup and seal that too. Keep away from sunlight or place the item in a foam lined box. This is basically the method NASA uses to store things long term. The only thing you can't really control is tin whiskers growing in the solder. If you absolutely need it to last, conformal coating the boards can help prevent the tin whiskers from touching anything else.