Create an ongoing system of collection and get written permission to use the testimonials for as long as you might need them. Have no time limit on them. Use testimonials from people similar to your prospects. The people you quote should be as much like your ideal prospect as possible. This increases identification and the feeling of relevance.
Testimonials also are more effective when they are from experts or people with congruous experience.
This is one reason you should collect lots of testimonials from a variety of people - you can carefully select targeted quotes when you need them. Usually I get her to sign a simple release although some people might not find that necessary. And a verbal agreement is usually adaquate. Use real testimonials . Don t try to rewrite or fabricate testimonials.