After his performance in the first debate, Kerry is now in a position to strike directly at the heart of Bush's negative campaign strategy. Specifically, he can take the charge that he's a "flip-flopper" off the table for the remainder of the campaign.
Without it, Bush has little left to run on but the sort of improvised negative attacks (i.e., those that do not fit into an overarching theme) that were Kerry's lot until recently.
Kerry set the stage in the first debate by: 1) coming across as forthright, direct and in command of the facts (and not fitting into the "flip-flopper" caricature); 2) addressing (albeit not as directly as he should have) Bush's claims that he "wavers."
What he needs to do now is really quite simple. In the second (or perhaps third) debate, after Bush accuses him several times of "sending mixed messages" or "changing his position" (or whatever the Bush campaign has decided is the accepted code for flip-flopping) he has to take the charge on directly and, in the sort of jiu-jitsu rhetorical move that he's quite good at in debate situations, turn the defense into an attack. And, if conditions are right, he should do it by directly addressing Bush (rules be damned):
"You've now said 11 times that I'm sending 'mixed messages.' But I haven't sent any mixed messages. I've been straight with the American people and I always will be. That "flip-flopper" charge was never true: it was something your campaign made up about me. Now, you can say it all you want, Mr. President, but that doesn't make it true."
This would have a number of effects: 1) in the debate itself, it will immediately throw Bush off his game, opening him up for a Reaganesque "there you go again" if he falls back into his repetitive talking points; 2) in the general campaign it could succeeed in making Bush's repetition of the "flip-flopper" charge and related attacks themselves into an issue: the president repeats these things because he has nothing to say; 3) it reminds people that Bush is out of touch with reality (the reality on the ground in Iraq, for example) and that the repetition of things that are "not true" is symptomatic of this.
Making the empty repetition of false charges into an issue will make it very hard for the Bush campaign, insofar as the entire enterprise is built on little more than these negative attacks.