A great article you should read and then apply the techniques mentioned.
You need proof to be more convincing.
As a creative freelancer, I mostly use #2, #5 and especially #6, which has the most amazing results. The before-and-after photos is my most persuasive technique that often leave my clients speechless.

6. Visual representations of results
Images are powerful. You can use before-and-after photos, charts, screenshots, and other visuals to prove that your product or service works and is worth the investment.

3. Press coverage.

While praise from a small-town newspaper might not do much for your credibility, a few words from a highly trusted magazine might be compelling and persuasive.

Read the entire story at: Copyblogger