3. My preferred strategy. Wait 5-10 years. If the movie still speaks to you, go ahead and buy it. The price will still generally be affordable, and may even be depressed due to people who followed #1. As Rich said, buy for your own pleasure and you won't go wrong. I won't get rich from this strategy, but I end up with things I like without breaking the bank, and limit the amount of 'back of closet' junk.
While you can probably find a Back to the Future 1-sheet for sale every week for the next 10 years, that is just not always the case with what the heart wants (right Rosa and her 3 posters?).
So if it finally shows up for sale and you can purchase it without breaking the bank, do it and enjoy it now.
I want a Jungle Woman insert, the last time I saw one for sale it sold for about $25 more than I was willing to pay. That was 10 years ago.
I may very well find it for less money whenever it turns up, but not only have I wasted a few minutes each week searching for it, I have missed out on 10 years of enjoying that poster. I was a blockhead to let it get away when I could have easily afforded it.
I would even apply this to Back to the Future. Let's say that for some reason in 10 years no one wants that title (doubtful, but let's say...) and you can buy it for $50 instead of $150. Is it really worth $100 to delay enjoyment for 10 years?
You wouldn't get $10 a year worth of enjoyment from owning that poster?
If not, this probably isn't the hobby for you.