Get advance buzz and signups going ASAP
We've already touched on it but it bears repeating: Get some sort of site up and start collecting emails as soon as possible. Pick your domain name and put up a logo and maybe a sentence or two that describes, or at least hints at, what your app will do. Then let people give you their email address. Now you're on your way to having a foundation of folks ready and waiting to be notified of your launch.