Janet,
Unless you have explicit permission from a client to take credit for the material, don't use any of it, or even refer to it with any specificity. The nature of ghostwriting is that the writer is unseen...hence the term "ghost."
What you can do is tick off your credits in general terms, making sure there's nothing identifiable, including time frame. Saying you've written a commencement address for an Ivy League University would be ok. Saying you wrote the Harvard University Class of 2008 Commencement Address would not. Saying you ghostwrote a memoir for a senator would be kosher; saying you wrote one for a sitting senator from NY would not. (That narrows it down to 2 people.) In any case, you cannot include any of the text or dates.
See if any of your clients might agree to let you cite them or give you a testimonial. Don't ask if you can display work or urge them to be specific about the particular work you did; assure them you just want something about the caliber of the work and your professionalism. You don't want to put thoughts in their head about you blowing their cover. Repeat customers will be your bread and butter. Also, think about what that would say to potential clients visiting your site. There'd be question as to how how closely you guard the client relationship.
There was a discussion in the Freelance Writing Forum a few days ago about this.
http://www.absolutewrite.com/forums/showthread.php?t=106247
Good luck.
--kch