Posters need to be well designed graphically in order to create the best impression possible.
Grab people's attention
Posters are typically presented in poster sessions, and you could be competing with hundreds of other posters for people's attention. If you have an image which shows your results, put it in the upper region of your poster and have it large enough to be seen by people walking by. If your results are impressive but there is no visual way to express it, have a title that catches peoples' attention.
You want your poster to be able to communicate its main point to someone passing by and spending less than a minute looking at your paper. Between your pictures, diagrams, title, and any graphs, someone should be able to have a general idea of what the main point of your poster is. Doing so will allow people to more easily determine that your poster is of interest to them.
Use a large enough font
People will be looking at your poster from a good distance. You main body text should be at least 24 points, and ideally should be even larger. You want all of the text on your poster to be legible from four or five feet away.
Avoid too much text
Too much text can be intimidating, and requires the font size to be too small. Aim for 800 or fewer words. Also, text boxes should be no wider than around 40 characters or 11 words.
Have high-resolution images
Graphics you put on your poster should have at least 150dpi (dots per inch). At this resolution the images should look sharp and crisp. At around 100 dpi, some blurriness can become evident, and at less than 100 dpi, pixelation can appear. Pixelated images look unprofessional, and look almost like you just grabbed the images off of Google image search.
Higher = more important
You should put the most important (or eye-catching) parts of your poster in the upper parts of your poster. In a poster session, crowds can make it difficult to see the lower portions of the poster, but the upper parts are usually visible. Additionally, when people scan a poster, they tend to go left-to-right, top-to-bottom.
Similarly, less important poster components should go near the bottom of the poster. This includes references, acknowledgements, and any logos you might be required to include.
Don't make it a set of slides
While it is easy to create a poster in PowerPoint by creating a 3x4 grid of slides, this creates a very uninteresting poster. You should design the poster as a whole, and spend the extra money to have it printed on a large-format printer.
Don't leave your poster
A poster session allows you to interact with people who are interested in your work (and who can give you many more ideas to work with). Have enough food and water beforehand so that you don't need to leave your poster and miss out on this valuable time.
Have handouts available
You want to encourage interaction with people who look at your poster, and you want to give them a way to get in further contact. Have available sheets with copies of your poster, a clear summary, and contact information.
Be ready to talk
Even though your poster should explain your research, you will be talking to people who come up to your poster. You should have prepared a one-minute description of your research, and you should be ready to go in more detail if people are interested.