Have your customers publish an online diary instead of giving you a testimonial. The diary would include regularly updated entries of how customers are using your product to improve their life. You could give customers a free product or a rebate in exchange for them publishing it online.
Your customers could write diary entries about goals they have reached using your product, the positive emotions it's given them, the fears and worries your product has taken out of their lives, how bad their lives were before they bought it, how it has helped other people in their lives, etc.
Your customers could update it daily, weekly or monthly. It will depend on how often they use your product. You could publish the diary right in your ad or link directly to the diary. You could provide the people with web space for writing the online diary or have them e-mail you the diary entries for you to publish.
You can make the online diary extra persuasive by also including customer's personal profile, pictures, online video of them using your product, net audio of them talking about your product, even scanned handwritten letters, etc.
An online diary would likely outsell the common testimonial because it's updated on a regular basis and reveals more personal information. A diary is considered private which would make people more curious to read it and believe its legitimacy.