Rolling a magnetic sign is one of the proper ways to store it but rolling can cause a permanent curl that will leave your once-flat sign with upturned edges. Rolling the sign in the opposite direction, to correct the curl, might be your first instinct. While this method works well with poster boards it can reduce the strength of the magnetism of a magnetic sign.
Unroll the sign holding two opposite sides, but not the corners. Pulling the corners of the sign could cause the magnetic material to stretch and degrade.
Place the sign over a flat metal surface. The side of a refrigerator, stove or filing cabinet will work.
Smooth the sign over the surface with your hands to ensure that the entire sign is completely flattened.
Set a blow dryer or heat gun to a medium setting and apply heat to the sign to help set it in a flat position.
Hold the dryer or heat gun about 6 inches away from the face of the sign and slowly direct heat over the entire sign. The heat will help to smooth out any curls.
TL;DR (Too Long; Didn't Read)
Consider storing your sign on the surface you used to flatten it, to avoid having to uncurl it each time you need to use it.
When applying heat to a magnetic sign, keep the heat at a medium setting and hold the hairdryer or heat gun a safe 6-inch distance from the sign. High heat and close range can weaken a magnet or even render it unusable.