Just had another idea:
If you guys will be at Mission Mayhem, ask teams to come for an open discussion or help session for your team. Its better when you can talk to the teams who are close to you on a one on one basis. Ask them to bring examples of their best sponsorship letters and just ask for help. It works better when you can actually talk to more than one person at a time b/c ideas develop better that way. And who knows, maybe some other team is having the same problems?
+$0.02
EDIT:
Here is the team list with their websites and such (Not 100% sure if that's for 2008). Try to contact some of them beforehand and ask them to bring examples of their best work for you guys. I'm sure they'd love to help.