Posters should be designed to convey one message, and not several messages. If you've got too much to say, you've got to cut it down so that you're not overloading the viewer with information and so that your message can readily be understood. This message must be obvious; viewers aren't going to waste time trying to figure out what your poster means, so your point has to leap from the page. If you need to explain something, such as in a presentation poster, use arrows to clearly direct the eye to the order in which things need to be read.
You should remember that a poster is a visual medium, so don't put too much writing on it; the visual effect of the poster should speak for itself, and the message of the poster shouldn't need explaining. A poster requires very concise visual storytelling in a limited space, and that story needs to begin and end on the poster and not leave viewers asking for explanations. Remember to include any important information, such as contact details, in a visible position.
There's no point putting together a poster that no one is going to look at. Whatever your poster is about, it needs to catch the attention of the intended audience. In some cases, this might require a striking piece of visual art, while in others, simply displaying a particular message in large print may do the job.
Onlookers need to be able to read the poster you've produced, and that means paying attention to your text size. According to the Center for Learning Technology website, you'll need to make your text size 48 point (at least two-thirds of an inch high) so that it can be easily read from a distance. Suitable fonts include Arial and Georgia, since these choices are clear and thus readily understood.
Don't be tempted to use too many colors, since this will confuse onlookers and prove distracting. You'll also want to avoid clashing colors, which will serve to lessen the aesthetic effect of the poster, and aim instead for colors that work well together. Remember that areas of white can be just as effective as more colorful designs.