Identifying a fundable research project
Whether you have a firm idea of the research direction you want to pursue or not, following the steps below will help you home in on a competitive research direction.
Read program solicitations to find the ones that seem closest to your research interests and goals.
Think about how your research interests and goals could support the vision and goals of the solicitations. (You may have to morph your research goals a bit or take some liberties with stated program goals.)
For the solicitations that fit you best, read all the abstracts of projects funded over the last two years and see if you can find an important gap that your research could fill.
Now it is time to talk to the cognizant program directors of the programs that fit you best. If your best fit is NRI, talk to me. For other programs, you will have found the relevant program directors during the previous steps. Contact them directly: NSF's staff directory. Not only will this discussion help you fine-tune your plan, but it's an important way to help research programs evolve to maintain their relevance and vitality, and thus is an important contribution to the research community.
In cases where the intellectual contributions of your project are significant with respect to two or more programs, you should contact program officers in both programs to discuss your project goals. If the associated program managers see the value of your project to their programs and know of the other program mangers' interests, they could agree to jointly fund a project that none of their programs would or could fund alone.
A perspective on the process
The steps above are a logical extension of what you had to do to choose a defendable PhD thesis topic; you searched the literature to find important gaps, you figured out important contributions you thought you could make, and you compared and contrasted your contributions to the state of the art. To have a successful career as a researcher, you need to extend the analysis of your research area just slightly, by being on top of current program solicitations and existing awards. Without doing this, you will have a very hard time "selling" your research proposal to a review panel.