Do not give your work away for free, friends. Repeat: DO NOT GIVE YOUR WORK AWAY FOR FREE!
I would write my own post on this, but for now here's the great illustrator Stephen Silver giving you what you need to hear. He directs it towards art students - and that's understandable because they're often new to the commercial world - but we all need the reminder because we can all be tempted to let our standards slip. I know I previously linked another of Stephen's videos where he gives similar advice - but it bears repeating because so many of us are intent on learning the hard way. Heed that man!
(Just saying - slightly earthy language, if you're easily offended by such stuff.)