Keep it simple
A simple design is an effective design. Don’t be afraid to keep white space within your layout: it makes content stand out even more.
Use a maximum of two font families
More than that is too busy, visually, for the reader to take in. This can make the design seem scattered, and detract from the layout.
Keep your brand’s image consistent
Consistency is key to attracting customers. If a brand doesn’t seem to flow, it confuses customers.