Send them an email informing them..."I know where you live. Please leave me feedback today. Thank you."
Moooowahahhahahahahaha! Okay, just kidding.
Send them an autoresponder about 14 days after the transaction. That's what I do and it works. As someone else mentioned, sometimes, no news is good news. Some buyers will leave no feedback rather than risk receiving a neutral or negative response from a seller in response to theirs.