Identify the intended warning or safety concept you want to the poster to convey. Safety warnings such as "Caution" are too basic for a designed poster; the word alone in bold print is enough. The idea here is to present a specific caution via a simple illustration, such as: Caution: Known Shark Area, or Caution: High Voltage. These words followed by an illustration of wavy lines to represent water and a protruding shark fin, or a few lighting bolts and a power line silhouette convey the message easily.
Draw simple illustrations. A busy poster with too many lines and colors will confuse the audience. Illustrations should be crisp, and their meaning should be quickly recognizable. Silhouettes are a perfect illustration to represent people, and can be modified to show people engaged in various activities, like a running man silhouette or a hiking silhouette. Other symbols such as a circle with a diagonal line crossing through it represent something you should avoid or not do. These are simple designs used to convey a message quickly, for safety.
Choose the size of your poster based on the degree of danger present, or the importance of the safety concern. Simply put, a minefield warning should be very large--about door-size--so that it's easily noticed. Whereas a "Remember to wash your hands after leaving the restroom" poster should be small, about legal or letter size.
Select attention-getting vibrant colors. Bright reds and cobalt blue are good eye-catching colors. Combined with thick boldface black type, these colors will compel a passerby to stop and read the poster. A poster that is hard to read or visually confusing will most likely be ignored.