A written testimonial should be phrased in a way that takes away a risk or neutralizes a fear. It should be phrased in a way that shows a value benefit or a specific enhancement. A testimonial should show action and make a call to action. "I used to use a competitor, I switched to ABC, so should you." A testimonial should overcome an objection. "I thought their price was too high, bought it anyway, came to realize they had the best value."
