You keep designs simple and effective by starting with a single, clear goal for each piece. Before any visual work, define the one primary action or message the user must understand. Every element you add should directly serve that goal. Remove anything that is merely decorative or doesn't guide the user toward that intended action or understanding.
Adhere strictly to a limited visual hierarchy. Use size, color, and spacing to create a clear path for the eye, typically in an F or Z pattern. Choose one dominant font, a simple color palette with strong contrast, and use ample whitespace to separate elements. This reduces cognitive load and makes the important information impossible to miss.