Ok where to start (I am opening this in word 2007 so if you are seeing something different then ignore some of my statements) These are the things that work well for us
Add your teams logo somewhere on the sheet even if you don't think you should trust me it adds personality to your request.
Have different levels of sponsorship with different benefits for each level.
I am attaching an old sponsorship packet that has served us well
2011-2012_Fundraising_Letter.pdf and
2011-2012_Sponsorship_Packages.pdf