With my usual blogging speed, I just finished listening to the NPR debate topic from last year, "Global warming is not a crisis" (aftermath reviewed by Gavin here).
Gavin had the toughest job, I think. Audience members understand that debaters will have certain biases and selectivity with the facts, but they also assume a certain level of honesty they're not going to get from the most prominent denialists. Gavin's role was to inform the audience that the other side isn't honest in its use of studies or facts, and that the lay audience doesn't have the scientific chops to sort through the dishonesty. The audience didn't like to hear that second part, but they need to hear it and they may ultimately absorb it after several hearings. I can say as a darn-well informed layperson that I get lost in some of the scientific arguments, especially the statistical ones, and these people who don't pay nearly as much attention will have to live with that fact too.
I'll add three suggestions: first, Gavin points out in his post that "the number of spurious talking points can seriously overwhelm the ability of others to rebut them." With the advantage of hindsight, that should have been part of the debate message, that the other side will use spurious points that the audience can't decipher on their own and that the pro-science side doesn't have the time to refute in the debate context.
Second, rather than attack the denialist side's credibility in general, the attack on their credibility should be directed specifically at the individuals at the podium for what they have individually done. Crichton and Stott particularly constitute Tim Blair-ish piñatas for this purpose. That's not a typical scientific approach, but the denialists aren't acting like scientists, and people deserve to know that. Maybe, eventually, they will rein in their claims if they find themselves confronted with them in different contexts.
Finally, I disagree with Gavin's conclusion that debates like this aren't worthwhile. Practice refines debate approaches. I know that means the bad guys also get to practice, but if we believe that truth has a tiny edge in persuasiveness over deception, then practice will ultimately favor the truth.