Lay out the poster on a flat, clean surface so the curled ends are facing down.
Place a piece of glass onto the poster, covering it entirely.
Weigh down the glass with a few heavy objects like books. Place these books at the corners of the glass, to keep it weighed down. You can cover the entire glass piece with books, but be careful not to break the glass or cause so much weight that it actually wrinkles the poster underneath.
Let the poster and glass sit overnight or for about eight hours.
Remove the books and glass. If the poster is still not flat, repeat the process.