lookig professional, (you already do I think).
Displaying recognised trust logo's.
There are organisations that allow you to display a logo if you meet their requirements for being a legitimate company and tht you fullfil your responsilities, and conform to things like the Data Protection Act, and that you keep any credit card transactions safe.....
I think the name etrust is one of those companies, but the fact that you havn't mentioned them and I don't know much about them (only they exist), could mean either that the users are more knowledgable than the designers in this case, if more users know about them thab designers; or that based on the high probability that we know more than users about what we design, not many users know about such companies.
You can tell you customers you keep all their information confidential and protect it form theft.
You can tell them you use secure connections when transfering sensative data such as redit cards (via SSL technology).
Those are, obviously, only if you really do, (no-one gains trust by lieing
).
You can even regester for a digital certificate for authentification that your site really is yours. Don't forget to renew it each time it expires else your users will get a warning - it will only way that the certificate has expired, but the damage is done when they read a warning related to your site, and authenticity.
*edit - sorry for all my typos and spelling, and lack of links tomore info - it's late and i'm tired!