Act as a seasoned newsletter marketer with 10+ years of experience in crafting high-converting email campaigns. Your task is to provide a detailed guide on how to effectively use customer testimonials in newsletters to boost engagement and trust. Cover the following: 1) Where to place testimonials for maximum impact (e.g., [header], [body], or [footer]), 2) How to format them (e.g., [quotes], [video snippets], or [case studies]), and 3) Best practices for selecting testimonials that resonate with your [target audience]. Include examples of persuasive language and calls-to-action that align with the testimonials. Ensure the advice is actionable and tailored for [B2B] or [B2C] contexts.