Just tagging along to what the other posters said...

I have seen website testimonials done in two ways that caught my attention:

1) a banner ad type box - with a different testimonial being shown per page as the user navigates site; very subtle and affirming

2) a "What Our Clients Say about Us"-type page. Where all comments are listed one after, sometimes in alternating fonts or colors for easier scanning/reading; all on one page packs a bigger punch.

An idea to solicit customer comments: When you bill them, somewhere on the detachable portion of your invoice, ask for their feedback with a comment box, and a request to use that information/ testimonial.