I stumbled upon this fantastic visual design guide that does a great job of describing the basic elements of design that make even the simplest brochure work or not. The lessons in this guide are applicable to any design task and will make sure you are prepared to get the maximum value from any design. I unfortunately Did not retain the referring site that allowed me to find this site, so I can’t give them a Hat Tip.
The basic cause of messy design is slight, unintentional differences among elements. This causes your brain’s visual processing to falter. First, your brain has to determine if there actually is a visual difference. Then, it has to determine the significance of that difference. Because the small discrepancies don’t actually signify anything, you end up wasting your audience’s brainpower. Strive for a consistent visual style where elements which are logically similar look similar to each other and look unambiguously different otherwise.