What should poster designers use to guide viewers and convey the main message?

Poster designers should use a combination of visual hierarchy, color, typography, imagery, simplicity, and space to guide viewers and convey the main message. Here's how each element contributes to effective poster design:

1. Visual Hierarchy: This is the arrangement of elements in order of importance. Designers can achieve this by manipulating size, contrast, and positioning. The most crucial information should be the most prominent, grabbing attention first.

2. Color: Using color effectively can evoke emotions and draw attention to key elements. Contrasting colors can make important details stand out, while complementary colors help create balance.

3. Typography: The choice of font and typographic treatment can significantly impact readability and the poster's overall tone. Designers should choose legible fonts for critical information and use size, weight, and spacing to differentiate sections of text.

4. Imagery: Pictures, graphics, and illustrations can communicate messages quickly and effectively. A relevant and compelling image can act as a focal point and support the main message.

5. Simplicity: A cluttered design can be overwhelming and confusing. Keeping the design simple helps ensure the message is clear and the poster is easy to scan.

6. Space: Proper use of space, or 'white space,' around elements can help prevent a cluttered look and highlight the most critical parts of the poster.

By carefully considering and combining these elements, poster designers can create a visually appealing and effective poster that guides the viewer's attention to the important information and makes the main message stand out.