Charts, graphs, tables, diagrams, blueprints, engineering drawings, and mathematical symbols and equations. You should use these visual devices when writing to engineers - for two reasons. First, engineers are comfortable with them and understand them. The best visuals are those specific to the engineer's specialty. Electrical engineers like circuit diagrams.
Computer programmers feel comfortable looking at flow charts.
Systems analysts use structured diagrams. Learn the visual language of your target audience and have your artist use these symbols and artwork throughout your ad, brochure or mailer.