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.
Tell the story of the problem that it will solve. Build a community of people who care to have that problem solved. Keep them informed of each step of the road to build the solution for them and get their help building it. Then when it's ready you have demand, a customer base, and a community that will attract others.
My firm went through this exercise and we have now moved from start-up to growth mode after two years.
We launched a highly disruptive real estate brokerage model and went up against incumbents such as Re/Max and Century 21 (deep pockets, market share, long heritage and well known). We quietly built and tested our concept in 5 markets. Once we had a story to tell went out with a megaphone and told the target audience nationally. We are now franchising nationally and have been named the brokerage of the future by the industry resource.
Defining the target audience was an easy exercise in our scenario. If you are launching a mass market product you will want to focus on a sub-segment that is manageable to reach. Build a teaser campaign for that audience and drizzle out benefits over a predefined period of time. Measure, tweak and repeat. Once you are dialed in it is time to go big.
Good luck and please let me know if I can assist. I donate 100% of my fee to JDRF in support of my nephew and other kids who are faced with the challenge.