I prefer to develop them on the fly, preferably using the person's statements back on them. That said, a lot of people aren't worth a well thought out retort, or they're just too stupid to understand them. That's when I pull out one of the following:
Whilst everyone has something to bring to a conversation, the only thing you should bring from now on is silence
or
Sorry, you've mistaken me for someone who cares
and finally
Go get a big black dog up ya.
Whilst everyone has something to bring to a conversation, the only thing you should bring from now on is silence
or
Sorry, you've mistaken me for someone who cares
and finally
Go get a big black dog up ya.