What should a sponsorship proposal include to get taken seriously?

A complete proposal should include: who you are and what you're doing, credible third-party data about your reach (Google Analytics, social media metrics showing followers AND engagement), demographics of your audience/tracks/series, what you're asking for, what you'll deliver in return, and how the sponsor will get a return on their investment. Mike DeFord emphasizes that sponsors receive about 50 sponsorship requests per week and won't do homework to research your series or social media - that information must be in the proposal. A single page telling who you are is not sufficient.