I used to work for a company (~50 people) whose entire business was based on the SBIR pipeline. We did a lot of super-interesting work!
Here's my advice:
1) Your proposal needs to be completely solid and well-structured:
- Describe the problem. Put it into a Defense/Intel context. Talk about the needs of the warfighter.
- Do a literature review of the field, and explain what the state-of-the-art looks like today.
- Explain what previous approaches to the problem have been attempted in the past.
- Demonstrate why those approaches are flawed.
- Describe your novel approach.
- Explain why your approach will succeed where others have failed.
- Talk about what you'll deliver in your Phase I deliverable, so that you can demonstrate proof-of-concept.
- Talk about how your eventual Phase II will put your proof-of-concept into a real-world scenario, and offer at least a glimpse of how your Phase III+ will commercialize.
- Talk about your team. Why are you uniquely capable of solving this problem?
- Talk about your budget. How will you spend the money toward satisfaction of the deliverables (salaries, subcontractors, equipment and supplies, etc)
2) As soon as you know you're interested in a topic, send an email to the Principal Investigator, telling them you'd like to meet with them to talk about the topic. Before the phone call, research the PI's history with this topic. Also, lookup the archive of SBIR topics, to see if this person has been a PI on similar topics in the past.
When you meet with them, ask clarifying questions that demonstrate you know the domain. Try to get as much specificity as you can... Ask them what their success criteria look like. See if you can get them excited!
Most importantly, by the time you submit, the PI should already know your name and to expect your submission.
3) If you're not already a recognized expert, with published academic papers on the topic, that's okay! But you'll improve your chances of winning a grant if you hire a known researcher as an advisor. For example, I've hired a Computer Science professor to supervise one of their own grad students, while doing paid work on a SBIR project. So the professor's credentials and the grad student's previous publications also became part of the SBIR proposal.
Anyhow, good luck!