I read many long articles dedicated to this very important topic. In nowadays no-one likes to read long articles. I decided to make it as short as possible and straight to the point. If you think I missed something, please let me know in comments.
1. You must prove your creditability. You have done it before.
2. You have impressive testimonials from your previous sponsors.
3. All of your previous projects were completed on time and a budget.
4. You have a strong story that will sell your film. The most important factor in the success of a low budget film is the script and the story.
5. You have distributors that will distribute your film.
6. You have a quality team members that worked with you on your previous projects.
7. Your previous projects met your sponsors/investors objective.
8. You have an exceptional communication skills and ability to build long-term relationships.