Since there are two states in which movie posters are originally issued by the studios -- either folded or rolled -- there are two different means by which they should be safely packed for shipment.

Folded posters: Folded posters are most efficiently shipped folded.  It is an excellent idea to package the poster itself in an appropriately sized plastic bag or inside of brown kraft paper (or even a surplus poster of no monetary worth) that has been taped shut. The wrapped poster should then be packaged between two or more thick and sturdy sheets of cardboard. Obviously the more vaulable the poster, the more sheets and the sturdier the cardboard should be used for packaging to keep the posters from being bent or mangled. It is a good idea to secure the plastic bag or paper wrapping to one of the sheets of cardboard and then tape around the two (or more) sheets of cardboard. For an extremely valuable poster, a sturdy box can be used to contain the cardboard wrapped poster and the remaining space within the box stuffed with light packaging material such as packing peanuts, newsprint, or plastic grocery bags.

Rolled posters: Rolled posters should be shipped rolled -- no exceptions (unless you want to incur the wrath of a condition conscious collector)! In the USA, the triangular tubes the US Postal Service provides free of charge for its Priority Mail service are inadequate for shipping-- here you get what you pay for and you will get an upset collector if you merely put a rolled poster in just one of these flimsy triangular boxes -- posters shipped in these boxes incur far more crushing damage than they should. At a minimum, a round cardboard tube should be used to immediately contain the poster (and then if you want to, you can put the round tube into a triangular cardboard box). The thicker the walls of the cardboard tube, the more resistant they are to crushing and if the tube's walls are thick enough, you can skip using a triangular box.

A clever trick some poster dealers use is to ship rolled posters in plastic PVC pipe which you can buy in the plumbing section at your local hardware store - I have yet to receive a crushed poster which was shipped in a PVC pipe! The pipe can be cut to the appropriate length, which means an inch or two longer than the narrowest dimension of the poster and end caps of the appropriate size can be bought or fashioned from cardboard circles.

When packing a rolled poster, be sure to pad the ends of the tube - a little balled up tissue paper, wadded news paper, a few packing peanuts, some scrap bubble wrap, plastic grocery bags, etc will prevent the edges of the poster from repeatedly bumping up against the tube ends and damaging the poster.

Also be sure to securely tape both ends of the poster tube - if one or both of the ends come off in shipment, chances are you will have a damaged poster and an unhappy collector on your hands.

The best poster packagers will secure the rolled poster in anything from thin plastic bag tubing (which is specially made for poster packaging) to brown kraft paper (in sheet or strip form) to another poster with zero value. This outer wrapper is taped (not the poster itself!) and further protects the poster.

If you are shipping both rolled and folded posters together in the same package, you should package all the posters as if they were originally issued rolled!

Well that's all there is to a good packaging job for movie posters -- it will cost a little more, but movie poster collectors understand this and will pay for the task being done correctly the first time rather than risk receiving a shipping-damaged poster -- the couple of dollars extra is super cheap insurance for both collector and dealer and is the best way to keep one's feedback high and one's customers happy!