We have an older sponsor letter on our website at
http://www.fsrobotics.org/moe365//te...003sponsor.doc
We do have newer ones as well as a trifold, double sided brochure (just printed in color on regular paper. On the outside is a listing of sponsorship levels and donations needed (supplies, food, services, etc.) with lots of pictures around it. On the inside is a copy of the sponsor letter. This is useful particularly when you talk to businesses in person.
In regards to sending letters, you almost always have to follow up with a phone call.