Don’t try to crowd too much information into a single presentation aid. Are your margins and borders large enough to provide ample “white space”? Is there anything you can eliminate? If the poster looks cluttered, consider making a series of presentation aids instead of just one.
If you use computer-generated graphics to produce slides, transparencies, or handouts, experiment with several different designs. Don’t get so caught up with what the program can do that you try to incorporate everything into a single presentation aid. If you do, you will wind up with something that is so “busy” that it will detract from your message, rather than enhance it. Be consistent in the use of background color and fonts. Simplicity should be your rule of thumb.