An effective poster is a visual communication tool.  

An effective poster will help you ... engage collegues in conversation ... and get your main point across to as many people as possible as quickly as possible.

Several people looking at a single poster.  There is text overwriting the image that says an effective poster is a visual communication tool to engage colleagues in conversation
An image with protesters holding up two signs.  Keep posters visual.  Use big text.  Further text adjacent to the image says and get your main point across, quickly.

An effective poster presentation ...

... is focused on a single message

... lets graphic elements tell the story, using text sparingly

... has a well-ordered and obvious storyline

An effective poster operates on multiple levels ...

... source of information

... conversation starter

... advertisement of your work

... summary of your work