Social Proof
The fastest way to demonstrate credibility is to let your existing customers do the talking. Nothing reassures a prospect more than seeing other companies, ideally ones they recognise, vouch for what you do.
But not all social proof is equal. A wall of generic five-star reviews doesn't carry the weight of one specific case study from a recognisable brand. A testimonial from someone identified by full name, title, and company beats one signed "Sarah M., Marketing Director." The quality, recency, and specificity of social proof matter more than the volume.
Examples: case studies, testimonials, reviews, user statistics, customer logos, awards.