Best way to pack posters?

I'm planning to move from the US to Europe this fall and will be moving my belongings in a shipping container. I have a bunch of posters and art prints, some framed, some not, and am trying to determine the best way to pack these for their 2-3 month journey.

Should I leave the framed ones in their frames and pack that way? Take them out and put them in poster tubes? Pack flat between cardboard? If I put them flat should I put a layer of paper or anything between them?