What is the best way to store posters?
      First off, the most important thing is where you don't store your posters. You want to keep them away from heat, humidity, light, and vermin (insects, rodents, etc). These are the three greatest enemies a poster has. So don't put them in the attic, cellar, or garage, or any place that's not climate controlled, and which is likely to attract vermin.
      I have spent much of my life in collectibles, and I can tell you that all the best preserved "finds" of collectibles that were ever made were found in dark closets that were in colder climates (like Canada and Colorado) that had been undisturbed for many years. Most of them had not been protected in any way other than being shut away in a cold, dark, dry place. The worst preserved "finds" have been found in warm climates (like Central America). The posters were shut away in warm, damp places, and they have major mildew and insect damage.
      So I advise you to get a filing cabinet (for folded posters and lobby cards) or a flat file (like a map cabinet, or you can make a large box out of acid free board) for unfolded or linen posters, and put the cabinet in whatever room has the lowest temperature and most stable humidity. I don't see any reason to put your posters in plastic bags (there's evidence that they may be harmful over time), unless you handle your items a lot.
      You can buy Mylar sleeves, which are wonderful, because they definitely won't hurt your items, but they cost a lot. If you store your items in a cabinet, your main worry will be theft, fire, or a broken water pipe, for which you'll need to purchase insurance. If you own your own home, it's easy to get a rider on your homeowner's policy at a nominal price (compared to the peace of mind you'll have).
      Just don't over-insure your items, and be very sure that you can prove what you had (don't worry about proving the value as you'll have to do that again after you make a claim). I suggest you make a video of your collection (or a list with pictures of every item) and store the video or list at someone else's place.
 
 What are the best types of items in which to store my collection (plastic, Mylar, nothing at all)?
      43 years ago when I started in comic books, I bought "poly" bags, and spent a small fortune over the next 11 years as a full-time comic book collector/dealer on bagging comics. Their main advantage is that if you handle your items a lot, you can do so much more easily, and with much less fear of damaging them, and your fingers are "oily" and can leave smudges on what you handle, even if you thoroughly wash them before such handling.
      However about 20 years ago, the Mylar and "Mylite" bags showed up, and the makers of those said that poly bags are damaging to comic books (or any paper items). I didn't rush out and buy all new Mylar bags for my stuff (which would cost a small fortune, as they are much more expensive than poly). Instead, I took everything out of the poly and JUST STORED THEM UNPROTECTED!
      I would say that I never saw any real difference between items I stored in bags, and those I stored with nothing (I've never used Mylar). With movie posters, I initially invested around $400 in a metal poster cabinet with four drawers that comfortably hold hundreds of unbacked one-sheets each (or around 50 linenbacked ones). I store all posters unfolded and flat, for over time they lose virtually all their folds (posters stored folded for many years invariably grow more fragile along the folds). Because this cabinet worked so well, I added more over time, and I now have six of them, so obviously I am a real believer in them!
      So my personal experience is that no protection works just fine (although I don't handle them virtually at all). If I want to look at items I frame them and hang them on the wall. Whenever I have received a collection to sell, it has never mattered how they were bagged or not bagged (and in fact the best condition ones that were ever found were totally unprotected, like the Royal Theater Collection).
      You just have to be careful about heat, light, humidity, and protecting them from water, fire, vermin, mold and mildew. Just about any well-constructed cabinet placed in a climate controlled room will accomplish this. I think the main reason collectors use the bags is because they think they are supposed to, which is fed by the endless ads from the companies who make bags. I know several other major collectors who don't use bags. I'd like some of the members of this club with large collections to share how they store their posters.
 
 A commonly asked question: How do I flatten out the crease marks/fold lines in my posters?
      You can often flatten out a poster by laying it under a board for a few weeks (see below) with weight (a lot of weight!) on top of it. It won't be perfectly flat, but it will help a lot. I use a large piece of flat cardboard to lay the poster on and then lay another piece of cardboard on top of the poster (making sure the corners don't get bent) and then put the weight on top of the cardboard. The cardboards should be larger than the poster otherwise the poster won't flatten out as well and the edges of the poster could get bent. And note that "newer" (post-1990) posters are printed on a different glossier paper that can take many months to completely flatten, while older posters will often flatten in just a few weeks.