The best option is to hang your signs as soon as they are received, as they will continue curing after they are hung.
If you should need to store them they will need to be stored on end like a book. If you lay the box flat for an extended period of time the signs will often stick together.
In addition, the signs should never be allowed to become wet while in the box. This will cause them to stick together and peel. Getting wet includes everything from getting rained on to condensation from being stored in a damp environment. Keep in mind that once the signs are hung moisture and wetness is not a problem. It is only when they are stacked that it becomes a problem.