I know when I first started collecting I took posters home by the armful from the theatre. I had a pine hope chest that's just sort of been an unwanted piece of furniture in my family for a while so I finally gave it a purpose and started filling it with poster tubes. I set up a spreadsheet on my computer and kept an overly detailed inventory of everything I had and even set what I thought were the going prices and felt like I was entering something very lucrative and prestigious.
After a short time I ran out of space in the hope chest and realized I hadn't opened a single one of those tubes since I brought them home. I invited a friend over to "help me inventory" just so I had an excuse to take them all out and show them to somebody. By the end of that day (and realizing that these posters were essentially worth nothing) I reverse engineered my collection down to about three posters. Two of those three posters were for animated movies.
For a while I didn't collect much of anything until on a whim I started searching eBay for posters from the 90s and early 00s. To my surprise they really weren't that expensive, and the artwork was usually very nice (Iron Giant sold me) and I decided I might want to buy some of these. But it was a slippery slope.
1. I'll only collect Pixar posters, and only the finals!
2. I'll only collect CG animated movies, and only the finals!
3. I'll only collect animated movies from 1991 (my birth year) onward, and only the finals!
4. Nothing is off limits. If it's an animated movie I loved and the artwork is appealing, I'm buying it.
It was an organic entry into the hobby. Working with mostly Disney taught me about re-releases, poster artists, etc. But with a near endless supply of contemporary posters showing up at the theatre six days a week, live-action had its appeal too. When I hit a financial wall with animation posters and the ones I wanted were simply too expensive, I started buying live-action posters.
If I may quote myself:
It is very uncommon for any two movie poster collections to be the same. Posters vary by country, by format, by time period, and be genre. Since the point of a poster is to hang it on your wall, people collect posters they want to display. This is unique to a collectible such as movie paper. Other collectible material like coins and stamps tend to be collected in series and sets, like Morgan silver dollars. While these collections are impressive, they are not especially diverse.
The big draw with movie posters for me has always been the individuality that comes with it. My collection can be literally whatever I want it to be and still hold merit. Mostly, I interpret the value of a poster. It's an entirely personal experience.
There've been a number of times where I've sold off posters I don't want anymore, but I think that's part of being a collector. I went through a brief phase where I wanted to collect lobby cards, and three sets in I wasn't interested more. So about $100 into it and I'm bored. You have to see what you like, what you want, and what you value.
It's not like coins where a book tells you that a Double-Die 1955 Lincoln penny is the centerpiece of any high-profile coin collection. That's up to you.