Posters serve as a great tool to share your research and as an advertisement of your work. Think of posters as giant, illustrated abstracts. You need to include enough information and data to keep your audience interested, but not so much that its difficult to read or follow.
This website, created by three American science professors, provides a comprehensive, yet concise, overview of how to create effective posters from defining your message, to planning your poster, to your poster's aesthetics, to how to effectively present your poster during the session. If you are only going to visit one resource on creating your poster, make it this one.
This website, by former evolutionary biologist and current photographer Colin Purrington, reads like a long, humourous, but exceedingly useful essay on poster design, both good and bad. He offers novel ideas for attracting attention to your poster, as well as a number of PowerPoint and OpenOffice poster templates.
This is a PDF of the CCMR's presentation on good scientific poster design. It includes tips for effective posters as well as examples of bad poster design choices.