@Lisa,
Be careful starting too early. Many companies will deny you if your event is more than six months away. The "sweet spot" so to speak, is between six months out and 90 days prior to. If you try after the 90 days until you will also get alot of no responses due to timing to accept/deny and ship. However, focus on your local businesses during the last three months. I have found they prefer to donate when an event is close.
One other suggestion, you may already do this, but let all of your donors know that they will receive recognition in your auction program, newsletter, website, etc. However many sources you have, let them know. The advertising is a push when someone is on the fence.
