Jason Segel's Weight Loss Motivation: Himself

By Rennie Dyball

06/24/2011 at 02:15 PM EDT

Some people count calories when they want to lose weight. Others turn to fad diets or start hitting the gym.

But for Jason Segel, who recently lost 30 lbs., it was his assistant – and a damning piece of photographic evidence – that did the trick.

"My lowest moment came [when] I shot a movie in New Orleans," the Bad Teacher and How I Met Your Mother star told David Letterman on the Late Show Thursday. "New Orleans is like, the best food and booze anywhere … I really took advantage of it."

PHOTOS: Stars' Workout Shout-Outs

So much so that one morning, his assistant found him passed out in his hotel room, covered in Taco Bell wrappers, his belly on full display.

"She took a bunch of pictures to show me what I had become," Segel, 31, says with a laugh. "That's my former stomach."

Quipped Letterman: "Honest to God, Jason, this looks like a crime scene!"