This video shows how to do a reverse roll to properly flatten a poster or print.
First, get a piece of craft paper and fold a crease at the end. Place one edge of the print or poster carefully underneath the fold of the paper. Roll the print using a poster tube or other round material.
Make sure that the poster’s edge is touching the fold in order to roll straight. Be careful to avoid creases or dimples in the art. Don't worry about it going really tight. That's not the key. The key is just to get it going the opposite direction of its natural curl.
After you get your print rolled up, tape the craft paper to hold the shape of the roll and leave it for several hours if not overnight. When you unroll the print in the morning, it will be much more flat. Thick paper might take longer to flatten.