You can always include a link in your email to the buyer that makes it easy for them to leave you feedback eg.
I've left you positive feedback - please take the time to leave me suitable feedback by clicking on this link:-
http://cgi2.ebay.co.uk/aw-cgi/eBayISAPI.dll?GetAndShowTransactions&item=0&nflw=1&page=1
{from recent post}
R2