The Sex and Cash Theory
This is credited to Hugh Macleod, a Scot who does drawings on the back of business cards. He also has a good piece on how to be creative within this same link. Check it out and it will make more sense. An excerpt:

THE SEX & CASH THEORY: "The creative person basically has two kinds of jobs: One is the sexy, creative kind. Second is the kind that pays the bills. Sometimes the task in hand covers both bases, but not often. This tense duality will always play center stage. It will never be transcended."

A good example is Phil, a NY photographer friend of mine. He does really wild stuff for the indie magazines- it pays nothing, but it allows him to build his portfolio. Then he'll go off and shoot some catalogues for a while. Nothing too exciting, but it pays the bills.

Another good bit from this site:

There are thousands of reasons why people write blogs. But it seems to me the biggest reason that drives the bloggers I read the most is, we're all looking for our own personal global microbrand. That is the prize. That is the ticket off the treadmill. And I don't think it's a bad one to aim for.