I can flatten anyone's belly, firm up anyone's arms," says David Kirsch. Furthermore, he adds, "I am the buttmaster."

A bold claim, to be sure. But Kirsch, 43, a New York City trainer, can produce testimonials—from clients like Heidi Klum, Faith Hill, Liv Tyler and Linda Evangelista—to back it up. Last year he landed the fitness guru job on Extreme Makeover, which in turn helped prompt his new book, The Ultimate New York Body Plan (McGraw-Hill $21.95). Speaking of posteriors: "I go to David when I need my butt kicked," says Klum, who credits Kirsch with helping her fit into a size 4 a month after the birth of her daughter Leni last May.

Kirsch is the first to acknowledge there is no magic bullet. "Exercise and diet—those are the real makeover ingredients," he admits. What differs about his book's regimen is the initial intensity: two weeks of a superrestrictive diet—no alcohol, bread, starchy carbs, dairy, sweets, fruits or fats—and 90-min. workouts every day. "My objective is to slap you awake," he says. (After that, the program gets easier.)

He does know something about change: Trained as an attorney, Kirsch fell in love with fitness, quit law and in 1991 opened his own gym, the Madison Square Club. Many toned New Yorkers are glad he did. Says Liv Tyler, whose first child is due in March: "I'm excited to see how he'll whip me back into shape once the baby is born."