Make sure the fonts are big enough for someone to read at least 3 feet away (4 feet is better). Use color. It makes your poster easier to read, and more appealing visually. Make sure to use color, thick lines, and textured lines (e.g., dashes, dots, stipple) on your graphs and tables.
Posters are often presented in a large room full of other posters during a 1 or 2 hour poster session. There may not be enough time for people to hear about each poster in detail, so people often "skim" posters from a distance to see what matches their interests. If a poster looks interesting, the person will come in more closely, and either ask for an explanation, or "eavesdrop" on an explanation that you are giving to someone else.
It is not unusual for people to be eating, drinking, and socializing during a poster session. Your challenge is to catch their attention in this distracting environment.
Recognize that your audience is trying to get as much information as possible in a short time, because there are many other posters to hear, too. You should prepare a 5 minute presentation that i) introduces the problem, ii) provides a quick overview of the 2-4 main ideas, and iii) provides a quick overview of your main experimental results and conclusions. Make it easy for your audience to get everything it needs in 5 minutes.
Keep your presentation simple. You don't want people to be confused at any time. Confused people drift away while you are talking, and won't remember you or your research. If they understand and are interested in your main ideas, they will ask about the details.
When you give an ordinary talk, the amount of information on your poster would be delivered in 20-25 minutes. In a poster presentation, you will move through the regions of your poster more quickly, just giving an overview of the 2 or 3 main ideas in the entire presentation. You will probably skip over material in many parts of your poster. If someone is interested, he or she will ask questions, and you will dig more deeply into the appropriate section to provide more detail.
Your goal is for as many people as possible to hear the 5 minute version of your ideas, and for a few people to be interested enough that they stay and talk to you for 10 or 15 minutes.
Sometimes nobody stops by to talk to you. There are many reasons. You may have a poor location, out of the main traffic flow. Your poster may be great, but on a topic that this particular audience doesn't care about. Or, your poster may be weak. Take some time to check out the other posters. Studying what makes other people successful will help you improve your own poster presentation skills.
