I second the "free for ___ months" then pay strategy. If a customer can't see the value in your offering after a few months of using it, they probably never will.

On the other hand, if you just charge customers off the bat, there are many people who can't overcome the activation energy of pulling out a credit card and will leave your site. I've come across quite a few services where even if they charged a penny, I'd probably pass just b/c I don't want to give you my credit card without knowing how useful your service really is.