Even as a solo developer you can use methodologies applied to large development teams.
- Write a Spec.
- Layout a UML.
- Do pencil-and-paper UI design.
- Hallway testing: If your expect a large crowd, ask Mom if it's easy to use.
- Peer review: You can build ad-hoc review teams with other solo developers.
- Keep an up to date schedule.
- and so on...
I solo develop all the time, and these practices keep me in line with my own work and give my bosses a great resource to know what I've done and how far along I am. And they keep me on track, to boot!