A lot depends on the service you're offering.

But I'd definitely include case studies of previous clients. Don't just offer testimonials but weave a story around your previous clients' experiences.

Again, I could help more if I knew what you were offering.

Another useful strategy is to offer some kind of special/limited deal on your service if they still don't bite after x amount of emails. Just be sure to move people from your autoresponder prospects list to a buyers list when they do eventually try your service.