Similar to El Greko's is one I used to use:
"I'd like to make a fool of you..." (looking upwards) "...but someone has beaten me to it!"
"The last time I saw one that size, it was hanging at the back of a donkey." (For really rude hecklers only).
When I was at college, being heckled by sports students:
"You can always tell the jocks - the ones with athlete's mouth..."