Alyice Edrich, who runs the Dabbling Mum (an excellent website for freelancers, BTW) presents in this article some important basic skills every freelancer needs. Among these, she mentions the need for a thick skin, some patience and at least a little humility. She also touches on query letter writing, invoicing and researching markets.
All in all, it's a great article for anyone who is considering writing on a professional basis.
Help! I Want to be a Freelance Writer! - Alyice Edrich
And if you'd like to see more articles of this quality, come see the list over at WriterSpot. It's a carefully selected group, with great care taken to weed out the fluff, the shallow and the obviously rehashed.