Hi everybody, this is Kelley. I would definitely recommend visiting the blog of creative agent Anna Goodson. She has several interesting articles, including "How to Become a Productive Working Illustrator in Today's Industry" and "How to Get Started as a Professional Illustrator." Her advice:
"I have worked with many illustrators over the years and I can tell you first hand that if you work hard, remember to always believe in yourself and in your work, don’t take no for an answer and enjoy what you are doing, you will definitely succeed in this industry."