One principle that is not always easily achieved is rhythm. 

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Rhythm in a design is important because it keeps the viewer stimulated.  Too much movement around a page can make your design too hard to read and cause the viewer to not see the message clearly. 

Using rhythm helps the viewer’s eye moving around the page in a way that the message is organized to be viewed as.  As we know, rhythm is a steady beat in music.  The same should be correlated in your design.  Using geometric repeating patters as a background is a good way to achieve this.  Using something as simple as arrows and call to action buttons, or line to create a lane for your eye to follow is a good way to give your audience a path to follow.  ("A Graphic Design Primer, Part II: The Principles of Design.")

Rhythm can be described a couple of different ways.  Fluid rhythm is a natural and organic kind of rhythm.  Nature has a lot of fluid rhythm in it.  Oceans, rivers, trees, and leaves are some examples of nature rhythm.  There usually is not a geometric pattern, but a natural free flowing rhythm that the eye can easily follow.  Progressive rhythm is our geometric or sequential rhythm.  It has a defined easily spotted rhythm to it.  A staircase is a good example of progressive rhythm. ("A Graphic Design Primer, Part II: The Principles of Design.")