New posters can take a while to flatten when you weigh them down with books or something heavy. Use the shipping tube they came in to uncurl them more quickly. Photo from Ryan McArthur.
You need the shipping tube your poster came in and a couple of rubber bands. Wipe the outside of the tube with a clean towel to remove anything that could damage your poster. Then, curl your poster around the tube with the design on the poster facing outwards. This should be the opposite direction that your poster came rolled in. Secure the poster to the tube with your rubber bands, but be careful since they can damage the poster. After ten or fifteen minutes, check to see if your poster is uncurled enough to be framed or hung up. If your poster didn't come in a shipping tube, you can use something else round like a foam roller, rolled yoga mat or roll of paper towels.
Uncurl and Flatten a Poster: A Step-by-Step Guide [Design Different