We in the industry don't put much stock in SEO/SEM 'certifications', because we know there isn't any magic bullet for either certifications or technique, but your prospective clients may and they are the ones you're trying to sell to. Most clients are so unfamiliar with SEO/SEM and they want some sort of 'assurance' that you know what you're doing before they hire you. Personally, I've been doing this work for 4 or 5 years, but have had trouble selling the service because 1)it's expensive and 2)I'm not as good at my salesmanship as I could be. I've also been looking into testing and
'certification' because I'm convinced it will help sell my services.

My second thought is this: get some hard data on the results of your work to really prove your worth. Ask for and make sure you get analytics and stats reports for those site you've had success with, then put that data into something understandable by your 'average' client.

And lastly, Peter was right: You KNOW how good you are. If you believe in yourself and your skills, it's so much easier to sell what you do.