Peeta's manipulation of the Gamemakers and the audience is based on his desire to help Katniss survive. To win favor for her, he publicly announces his love for her, influencing the audience to sponsor her and provide her with help. When the Games begin, Peeta aligns himself with the Careers, knowing that they will not kill him until they have killed everyone else; the longer he can survive, the longer he can help Katniss. As the storyline of Peeta and Katniss begins to garner audience favor, the Gamemakers take advantage of it and announce that two tributes may win the Games this year if they are from the same District. This allows Peeta and Katniss to focus on keeping each other alive. Finally, Katniss shows that she has learned from Peeta at the end, when the Gamemakers try to reverse the new rule; her deliberate attempt to deny them a single winner is the same sort of manipulation that Peeta has been practicing, although his is based more on his feelings for Katniss than on rebellion.
Peeta uses his honesty and his general goodness to manipulate the Gamekeepers. He makes choices like teaming up with the "careers" at the beginning not to ensure his own success as the gamekeepers would anticipate, but to ensure the survival of Katniss. Peeta also uses public announcements pre-games to manipulate the public and gamekeepers, for example declaring his love for Katniss publicly winning himself a great deal of sympathy. In the book Katniss notes herself how Peeta's self deprecating humor and general niceness win him respect.
When Katniss and Peeta is left they both start to swallow the posionous berries. But than the people had to have a winner so they bboth survived.