One universally accepted thing to avoid is permanently adhering a poster to its backing. From the 1960s – 1980s it was popular to have posters dry mounted to a board so that they would lie flat in their frames. This process is (almost) irreversible and posters that have been dry mounted are considered by the vast majority of collectors to be both compromised and diminished in value. How much exactly depends on whom you ask but a good estimate would be circa 50% of the value of one that wasn’t dry mounted to a board. In short, don’t do it!!!
With the above disclaimer, I have been known – on occasion – to purchase a dry mounted poster if offered to me. Why? Simply because the particular poster was extremely rare or hard to find and I was unsure if I would be able to find another example. Luckily, I have a great working relationship with a gentleman working out of Chicago who must be – to the best of my knowledge – one of the few experts in the poster restoration field who can safely remove most posters that were dry mounted or glued to a backing.