These are some helpful hints on how to create a visually appealing poster.
Colored backgrounds
Diagonal color blends can be a problem, Horizontal or Vertical blends seems to be OK. Some PowerPoint Templates are better suited for posters than others. Try using color that compliment each other which is more pleasing to the eye. Also, make sure you use colors for your background that will make your text pop, not fade. Example: yellow text on a white background, yikes!
Textures
Most patterns and textures don't work well on posters. They are usually distracting. You want a background that works with and emphasis your information, not a color or pattern that will make it confusing and hard to read. Try using large blocks of color to emphasis information or a pattern that works with your text and graphics.
Images
Use high quality images! Web GIF files usually are not of sufficient resolution for posters unless they are used with very little enlargement. Use good JPEG, TIFF and WMF To verify if your image will print with enough resolution, zoom in with your view control to 100%; if the image is very course, that is how it will print on your poster.