When I was a marketing consultant, to win work I generally had to write a proposal of some sort, outlining what I would do, by when and how much it will cost. The better the proposal the better my chances of winning the project.
I was going through a stage where I wasn't winning as much work as I wanted to (or needed to). I had some testimonials, but I felt that my proposals needed something more. So I decided to really be open and honest and give my prospective clients access to my most recent clients.
I started to list my last ten projects along with the client's direct contact details, and with their permission, I encouraged the prospective client to contact each and every one of them. I explained in the proposal that these are not vetted contacts, the list is simply updated with the latest ten projects done by my firm.
Now this is much more powerful than testimonials alone--it is a real time, current and accessible list of people, prepared to share their experiences of working with me, and I didn't really know what they are going to say.
You can't be selective and just pick those clients that you know will say nice things, especially if you haven't done work for them for sometime, because any smart person sees through this in a second.
So the bonus here is that you have to really make sure you are delivering a high level of service and value for money to each and every client, and that is just good business.
I immediately started to win a lot more work, the value of the work increased and the feedback from the new clients was that they really liked the open approach to sharing my contacts for the most recent projects, not a meaningless testimonial from a project I did five years ago.
In business, credibility and trust come from putting your money where your mouth is, by being open and transparent.