Self deprecating humor, while stinging your opponent, always works...

THE PRESIDENT: The nonpartisan National Journal analyzed his record and named John Kerry the most liberal member of the United States Senate.

AUDIENCE: Booo!

THE PRESIDENT: And when the competition includes Ted Kennedy, that's really saying something. (Laughter and applause.) I'm telling you, I know that bunch. (Laughter.) It wasn't easy for my opponent to become the single most liberal member of the Senate. You might even say, it was hard work. (Laughter and applause.)