There can be frustration in trying to flatten a poster, but there are a few tricks to make it easier. The first thing to do is assess the situation. Whether the poster has been rolled up or just curved on one end there are possibilities. Mounting board is one way to keep a poster flat. If it will need to be rolled again, then there are options to take of the problem as well.
First of all, if you are using mounting board, one may be purchased at most hobby or home supply stores. It comes with or without adhesive, but it needs to be about 1/2 inch larger than the poster. It can be trimmed if a frame is not going to be used. They generally come in standard sizes in order to accommodate most posters.
Put the mounting board on a table or other flat surface where you can leave it during the process. Lay the poster on top of the mounting board. If it has been rolled put something heavy on the edges, books are a good option. If there are places that are curled or wavy, you can use a rolling pin from the kitchen to ease out the kinks. This may take a little pressure but don't push to hard or damage may occur. Also, it needs to be clean and smooth.
Once you have the poster in place, you can place small clamps on the sides to hold the poster in place. The clamps can be as simple as paper clamps, or clothes pins. There are artist clips but these are more expensive, and many not be necessary if you are only going to use them once. If there any areas that are not perfectly flat, then turn your mounting over, and weight it down with heavy objects to cover then entire area. It might be helpful to put a sheet or tablecloth down before turning. The fabric will keep a speck of dust or furniture polish from getting on the poster. Wait anywhere from eight to 12 hours before turning. It should be flat and ready to frame or hang.
There are also poster hanging kits that may be purchased from hobby shops. One contains mounting board, four angle clips that fit across the edge of the corners, and adhesive. Another kit contains a cardboard backing, four corner clips, and four plastic edges that look like a frame when attached. The cost for either of these is moderately priced.
Be sure to keep your poster rolled in a tube until you are ready to use it, in order to prevent damage. Once mounted, keep them out of direct sunlight to keep them from fading. Even in a frame this is a good idea.