1. Beauty: Most poster artists during their careers designed posters that are considered to be outstanding in their beauty and appeal. While artistic merit can be very subjective, there is a general consensus on most posters as to the merit of their beauty.

2. Subject Matter: Event posters like the Olympics are collectible. Certain subjects like movie posters and travel posters seem to always remain in demand.

3. Rarity: Some posters are seldom available for purchase. Very likely there are but a few remaining. These posters may be in museums or private collections. When rare posters such as these become available, collectors will compete to own it. Conversely, if a poster is relatively easy to find, this will keep it's price from appreciating.

4. Artist: Posters by famous and highly apreciated artists are always in demand. Mucha, Toulouse-Lautrec, Cherét are a few that fall into this category. Today, the most collected posters are by Leonetto Cappiello because he was so prolific. Many of his poster designs are still available.

5. Condition: Obviously, the better the condition, the higher the value of a given poster. Learn more about determining poster condition.

6. Dealer prices: It's a good idea to keep abreast of current prices, for the poster in question, and for other, similar posters. This will help you to develop a well-rounded perspective on poster prices in general.