I do a lot of contract work. What letters should I get from my clients, other than MoU and invoice, which would serve me as proof of my experience for future jobs?
If the MOUs include a listing of your deliverables and the listing is explicit and specific enough, this should be enough proof that you know what you are trading in - You'd have been out of a contract if you had failed to deliver.
If you have your own website - you could create one for yourself for free on say tumblr.com, you could include some testimonials from the clients (cite the name and the company on the page but leave out their contact info) you worked with about the specific skilled tasks you accomplished for them and how well you came through. And now that I think about it, LinkedIn also gives you the ability to post these client testimonials.