First, and foremost, if you're including the word "expert" in your own bio (i.e. self named), stop. It's a big turn off. If you have a quote from a client that calls you an expert, that's fine.

Second, get referrals. Word of mouth is key.

Third, publish case studies and reports as part of your client profile. You don't have to include hard numbers. You can publish percentage increases of opens, clicks, conversions, etc.