Make it easy for people to provide testimonials. One of the most common comments you'll hear from clients when asking for testimonials is, "Well, I'm really not much of a writer, so it's hard for me to put it into words."
Source: Colleen Francis, sales trainer/consultant
The real power of testimonials comes from the fact that they're not polished ... they're authentic and from the heart. Show the client other client testimonials as an example. That should set the table for them.
Next, ask your client the following: "Finish this sentence in 25 words or less: I really like (product/service/person) because ..." This really works because it gets right to the point about the feelings people have for you, for what you do and for what you're selling.