I have been asked to include a few sample letters. Hope these help.
Create a letter like this, and give it to the teacher to send out.
Sample Room Mom Introduction Letter
A month before the party, I send out a letter like this.
Sample Party Flyer: Request for Volunteers
At least two weeks prior to the party, I send out this.
Sample Confirmation of Volunteers
Then, two or three days before the party, I send out email reminders.