• poster by theme such as transport, sport or theatre
• posters by a particular artist, such as Fougasse or McKnight Kauffer
• posters of a particular country, such as France or Switzerland
• posters of a particular style, such as Art Nouveau or Art Deco

"Anyone beginning a collection should also spend as much time as possible visiting auction houses and dealers to examine posters closely. However many books you read, there is no substitute for 'hands-on' experience; handling the genuine article is by far the best way of learning to spot reproductions and fakes." The provenance of posters is important, but "provided collectors but from a reputable source that guarantees the authenticity of the poster, they should have little to worry about".

Specialist poster dealers and auction houses are the most common source for purchasing posters. Auction house catalogues offer a wealth of useful information, most commonly the name of the artist (if available), the printing technique, the date of printing, the size, and a price guide (although this cannot be predicted with certainty, and does not include commission + VAT). Many catalogues also give details of the condition of the poster.

The key factors affecting the value of posters:

• The reputation of the artist. "By far the most expensive posters are those by Toulouse-Lautrec, since their value is boosted by his reputation as an international artist."
• The rarity of the posters. (Alphose Mucha, the Viennese Secessionists and rare Russian Constructivist posters can receive very high prices). Posters are a mass-medium, originally printed in large numbers, therefore limited availability pushes the price up. It can be difficult to establish exactly how rare a poster is, as, for instance, Russian Constructivist posters were very rare until the collapse of the Soviet Union, after which exports appeared in the Western markets.
• Signed works. There are many unsigned posters (and those by lesser artists) which can still fetch a good price, although the value is "largely dependent on the quality of the design and the commercial appeal of the image". Posters depicting sports, vintage cars and fahionable figures, or those that reflect the mood of their times are in high demand, even if unsigned.
• The condition of the posters. Most posters were printed on inexpensive paper because they were not designed to last, and most will have suffered some wear, although some will have been restored. You will need to ask for expert advice as to the value of posters.

Gleeson advises that "provided you buy from a reputable souce and collect only posters that genuinely appeal to you, rather than an investment, your collection will give you great lasting pleasure - and any increase in value will be an extra bonus when you decide to sell."

Information extracted and summarised from Janet Gleeson, Miller's Collecting Prints and Posters, 1997, pp.92-93

I still hope to get some information for this soon from a vintage poster dealer (as it's not really my field of expertise). Meanwhile you can find some information on the vintage poster stores page, or use the Miller's Collectors Guides.