I keep my rolled posters in tubes and my folded posters in flat files, so I'm not necessarily an expert, but one way to flatten a rolled poster is to roll it the reverse way and hold it tight for a couple of minutes. It usually comes out pretty straight. That's what I do if I have to take a picture of a rolled poster. To keep folded posters flat, I use Emovieposter books. They are the perfect size and weight. I probably have 100 of those books, so I use them in every drawer, at every corner. Then stack your posters on top of each other, you can probably stack 300 posters per drawer, and they will eventually all be flat.
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