This New York Times short of Jerry Seinfeld revealing how he wrote a particular routine is revealing:
If you thought just because it sounds effortless, it is... well, you'll see!
You might also enjoy this compilation of Jerry talking about the elements of success as a stand-up comedian and other aspects of life as he's interviewed by, among others, Barack Obama:
My favorite is his conclusion that you have to "find the torture you're comfortable with." Meaning, everything worth doing has aspects that are not fun but to get to the good part you have to be willing to do the other stuff.
Also interesting is the fact that when he had a meeting about his online show, Comedians in Cars Having Coffee, they all told him it wouldn't work because nothing longer than five minutes succeeds. His episodes average nineteen minutes and the series has been a big success.