Paul Krugman's a good example, because he goes on the offensive, but he's not quite offensive enough. For good, juicy ad hominems, read SLOG (the blog of the Seattle Stranger) or Feministe.

How to offend a conservative is an interesting question. I think it should be easy to offend (or upset or disgust) a traditional social conservative, with simple sexual shock value. It's harder for me to think of ways to offend an economic conservative. The closest thing I can think of is stereotyping libertarians as weirdo losers.