Baby come back!

After a chat with your friend, when he/she departs, suddenly call him/her back. When your friend comes back, act as if you're thinking and say, "Errr... nothing."

Alternatively, when your friend comes back, say "I forgot to say bye. Bye!"

Do this when there are a lot of friends around you. The more the number of onlookers, the more the irritation of the victim.