Measure the poster that you wish to preserve. Measure at least twice to make sure you are getting the correct measurements.
Extend the measurements by one inch on all sides and then cut a piece of stiff cardboard to the larger measurements using a utility knife. To make sure that the cardboard is cut to its proper size, place the poster in the middle of the cardboard and then check to see that there is a one inch border all around the poster.
Spread out a piece of .5 millimeter thick plastic wrap on a large, flat surface. Place the poster face down on the plastic, and then the stiff cardboard on top of the poster.
Pull the plastic wrap around to the back of the cardboard, and then tape it in place using strong packing tape. Do not pull so hard that you bend the cardboard, but pull it tight enough so that the plastic on the cardboard holds the poster in place. Do not get any tape on the poster.
Lay the plastic-wrapped poster face down in a flat poster storage box. You can purchase flat poster storage boxes at almost any storage or shipping facility. You can also check the Internet poster preservation websites for flat storage boxes.
Store the box in a dry and cool place on a flat surface. Do not place anything on the storage box.