Marketing can make the difference between a few downloads and thousands. There is no secret formula that guarantees success, but there are things you can do to increase your chances. Although marketing is covered in prelaunch, it really spans the lifetime of your app.
Before launch, the focus is on building infrastructure and interest. Infrastructure adds app functionality for easy rating and tell-a-friend functionality. Word of mouth can be a powerful and low-cost marketing tool.
There are many open-source projects for reminding a user to review and tell their friends. You can use them directly or as a basis to write your own. (And remember to contribute back if you make it better.) These are a few of the common ones: