Audience
When designing your poster, keep reminding yourself of your audience. Who is your audience? Are they a general audience with little knowledge of your subject? Do they have some knowledge of your subject, but not specialised knowledge of your particular area e.g. presenting to your department or a broad conference? Are you presenting to a specialised audience who have an in-depth knowledge of your subject matter?
What will interest these different audiences? For example, a general audience may be more interested in the real world application of your research, whilst a specialised audience may be more interested in the technical details in addition to the significance of the work.
What type of language is appropriate? A general audience may not understand the jargon used in your field, whereas a specialised audience will most probably be familiar with technical language.