The best way to make a prelaunch is to make a prelaunch for prelaunch (a pre-prelaunch).

Prelaunches always works best if you already have a list (if you don't have a list, don't worry, you can always find partners that are willing to do business with you).

From my experience you should start softly promote your product way before you are planning to do a launch. Send your list a quick email that you are making "something good". This will start a small buzz, people will know that you are up to something. After some time you can give them few pieces of puzzle, give them more details about what you are making, until the whole puzzle is revealed that means, when you enter prelaunch stage.

On prelaunch people will know exactly what your product is about. Prelaunch is mainly bunch of emails reminding when the launch is and giving the free stuff out (video training works best in from my experience).

This is just a rough prelaunch concept that I used for my launches. Those launches were off internet marketing niche, but I guess you can use this concept for any niche, you just need to change the angle a bit