You should attempt to find a special flat box or glass carton sized specifically for your mirror when purchasing boxes. With the proper box in-hand, you should prepare by lining outer edges with crushed paper to create a padded bed in which your item can rest.
Before you place the mirror in its box, you should wrap it completely in padded paper or sheets of bubble wrap to create an ample surface barrier. Tape this wrapping tightly and place your mirror in the box.
Once in the box, fill all gaps with crumpled paper. Although it is not normally recommended, you may attempt to pack more than one mirror in a single box. If you choose to consolidate items in this manner, be certain that you’ve provided sufficient padding between items to prevent rubbing or shifting.
Heavy mirrors, stone or table tops
It is generally recommended that heavy glass and stone be professionally packed into specially crafted wood crates to provide an extra measure of protection. Your United Van Lines agent can provide this service for you.