well, I always feel that the best way to convince someone from the left is to use someone who makes your point come from the left. For example, any article by Naomi Wolfe or Glen Greenwald are always a good read. Politicians like Dennis Kucinich or Ralph Nader are awesome at calling out the President on many bad policies, and points out how hypocritical liberals are for supporting him. And Jon Stewart is hilarious and often takes libertarian stands. Read the comments section of the segment he did on the New York soda ban (you know, for having too big of a cup), and most of the people criticize him and defend Bloomberg.
Use people associated with the left, and they may be more willing to take your conservative or libertarian position seriously.
liberty lover in Nor Cal!