I wrote an article on this somewhere (might be on my site if it's not here).
In short, I think the best way is to date the old fashioned way and tell her early in the relationship but not right at the very beginning. Telling her the right way and at the right time is what it all hinges on. I described this in more detail in an article on my site.
I had some CDs tell me that they had success with online dating but they were honest and upfront about their lifestyle from the start. Sounds like it's worth a try.