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Theory:
We are likely to value third-party consultation as objective, confirming, and without motive.
Application:
When we read reviews of 3rd party experts/ consultants, we value that information as being more objective and trustworthy. So besides testmonials of your customers (see Social Proof), can you get some external experts to review or talk about your services on conferences or their blogs? Even when you pay them to write a review on their blog and tell the audience it'll still work (example: sponsored review on Yoast.com). Of course it helps a lot if the review is positive ;).
