Think about it from their point of view. Then, address that in your sales letter. Remove anything that would keep the reader from making the purchase. Share actual excerpts from what your current customers are saying about your product or service. Many websites have an entire section or even a separate page that has endorsements and compliments listed.
Satisfied customers remove some of the doubt in the mind of the reader.
If these people have found a lot of value and benefit in the product, then I probably will too. A headline isn't just relegated to the beginning of your ad copy. Use them frequently - but don't overuse. You will want to spend as much time working on your headlines as you do the entire sales letter. They are that important.