Many of the tips and the overall philosophy on this blog are relevant to designing a poster.
The first thing is to consider the audience for your poster. Do you expect someone to read the poster while you're not there, or do you want to use the poster to support your oral presentation? To me the answer is obvious and it is the latter. Most poster observers are walking around, they don't want to stand there for 20 minutes reading alone, and they want to be guided through the substance quickly. Moreover, you can always have a handout with more detail
Like a slide presentation, don't use a poster to replace your journal article.
Below is an example of a 36" x 48" poster that supports my activities on this website (it might be too small to read as reproduced here, but you get the idea). I've limited myself to 9 fairly simple visuals and provided enough simple text to help me talk a listener through the content and even to help a viewer if I'm not there. Just like preparing a presentation, you need to think through the key summary points and not overwhelm the audience with detail. Exclude all graphics that you don't plan to talk about. Be sure you can talk through the poster in 5 minutes, 10 at the outside. Remember, the idea is to get people interested in your work, not to give a comprehensive review. Show the most important results on easy to understand graphics.
My presentation first walks the listener down the left most (blue) column, summarizing the problem statement, the overarching question, and the overarching solution. The top gray horizontal bar provides more detail about the challenges, while the tan box provides two examples of converting bad slides into good ones.
I can get through this in about 5-10 minutes. If the listener is interested, I will direct them to more detailed information.
Keep your poster simple and clean. An alternative to the above format is to have the poster center on a single overall "systems" diagram of your research (see October 15, 2015 "tip"). A single central figure can be a great device for a poster if you can use it to talk through the essence of your research.