What is the best approach for sending sponsorship proposals to potential sponsors?

Don't make specific recommendations in your proposal until you've gotten to know the company and understand what they actually need. Your research may give you ideas, but you don't truly know their objectives yet. Additionally, don't treat sponsorship pursuit as a one-and-done effort - it requires multiple touches over time. One young racer sends cards throughout the year updating on race results and stops by at PRI regularly. It's these repeated touches that make the difference.