Hi Natasha,
I'm not sure about ratio, but I feel like our Facebook page really helped here. Every time a parent donated something awesome, we talked it up like crazy. "So excited to see the brand new Tivo, donated by another awesome parent.." type of stuff.
Also, be VERY specific in your requests. Tell people exactly what you need, instead of just a generic, "anyone have anything to donate." Communicate via constant contact and your website at least once a week.
Success begets success, so feel free to fake it a bit. Start your communication with "many thanks for the awesome items that have already come in. Your generosity is seriously appreciated. For those of you still looking for a way to contribute, we are in need of sports tickets, restaurant gift cards, and X and Y..." It makes people think "all the cool kids are contributing. I had better."
As for class baskets, we write 27 (the number of classes) themes up, then send the theme to each class parent, and send them the communication each class parent is to send, as well. For example, "Mrs. Brown's class theme is "Green Thumb.' Here is your letter: 'Hi Mrs. Brown's class. We are collecting for our school auction basket. Our class theme is Green Thumb. We are asking for a contribution of $5 to $10 per child, from which we will be buying gardening supplies, books, etc., and putting together our basket. If you would rather contribute a gift card, please feel free. It is due to me by Friday the 12th..."
Anything less specific and we hear crickets too. HTH