I have found that the best way to create demand for a product that does not yet exist, is to capture the interest of people who experience a common pain that *does* exist.

Focus less on selling the non-existent product, and focus more on clearly articulating the problem your product will solve. And, make it extremely easy for people to:

(1) Tell you they have that problem too.
(2) Realize there are a lot more people like them.
(3) Evangelize a potential solution to all their friends.

I am extremely biased, but http://launchrock.com is a great way to start capturing this list, and enabling them to share.

Look at Mailbox.app and beachmint.com for inspiration.