For a single developer, I usually recommend two things:
First, look at the Personal Software Process. It's documentation and statistics heavy, but it's one of the very few formally studied, researched, and applied processes for individuals. Most of the work has been done toward teams of at least 5-7 people, and on top of that, frequently in the context of a larger organization. The PSP is designed to allow a single developer to examine his/her processes and methodologies to determine where they are being the most effective and where in the process problems might exist. However, the PSP doesn't say how to do anything - there are no suggestions for how to carry out the steps, but rather just the steps that should be carried out and how to review your methods used to carry out each step.
Second, look at commonly accepted "best practices" and see how they fit with the product you are developing and how you work. Since you are working solo, there's no need to accommodate other developers. This means you can choose everything from how you get things done to the tools that you use to support those methods. You might want to determine if and how you can use or define practices such as unit and acceptance testing, "you ain't gonna need it", release scheduling and milestones, coding standards, and capturing requirements and design decisions (just to name a few of the big ones - there are more).