I keep a file of testimonials, all of which are unsolicited. The best way to get truly enthusiastic and realistic sounding testimonials is to provide service that is so good, people take it upon themselves to tell you how much they like it.
When they do, just ask them if you can use their comments in your marketing materials. Usually, the answer is "yes".
One other way to get them is to provide a questionnaire about your services... ask some questions about how you can improve. Ask what they liked and what they disliked. The good thing about this approach is that it allows you to tune your services to what your customers really desire.