I'm going to make a suggestion. I am a buyer on ebay and I must admit I sometimes forget to leave feedback however when the seller ships my purchased items in a timely manner and sends me an email or includes a letter in my package saying something like "I have shipped your item(s) today. When you receive your item kindly notify me either by email or leaving positive feedback. If you have any concerns please don't hesitate to contact me."

For me this is an added personal touch that shows the seller believes in the products he/she is selling and cares about me as a customer. I am always more inclined to purchase from the seller again and I always remember to leave positive feedback for him/her. 

Timely and polite responses to buyer questions always helps too when wanting buyers to leave positive feedback.

I know this won't work for all buyers, but it might decrease the number of buyers that don't notify you or leave feedback when they receive their item(s).