There are many security standards you can "certify" your datacenter with, such as NIST or ISO. You can read more about them here: http://en.wikipedia.org/wiki/Cyber_security_standards

In addition there are some SSL and Antivirus vendors who provide "security badges" that can be embedded to your website and prove that your DNS domain and SSL certificate are valid.

Having said that, If you are serious about security and want to leverage that as a competitive advantage you would probably have to implement deep security measures.
You could then publish these measures in a whitepaper that can be shared with your clients.

You can refer to what Amazon have published regarding their AWS product: https://media.amazonwebservices.com/pdf/AWS_Security_Whitepaper.pdf

Guy