While looking at something else, I came across 3 Things I wish I'd done by Jeanne Sahadi. While Jeanne writes about money and finance related stuff, I think the third point about valuing your work is right on.
Early in my career, I worked for a very large university in their IT department. Just being out of college, I was very grateful to even have a job. But anybody who works in the public sector knows, the pay ain't that great. In order to make ends meet, I started writing code "on the side" with a friend of a friend. I wrote a system in Paradox (version 3.5 for you techno geeks out there) for $10/hr. I got paid, but found out later that the person I was working for billed the client $50/hr for my time. This person then went on to sell this system to other clients and, of course, I didn't have any rights to it.
I also wish I listened to my gut. I had been working at a regional telecom company in the late 1990's. I had been perfectly happy there for three years and the pay was more than adequate. In 1998, during the rise of the dotcom bubble, I was seduced by a consulting company by the promise of 100% bonuses and four day work weeks. I agonized about it for about a week, and although my gut told me it was too good to be true, I jumped ship. After about two weeks I knew I made a mistake, but told myself it would get better. Fortunately, I had a good management team back at the telecom company and kept in touch with them. After two months I decided I'd had enough of consulting and went back to my old job.