Robert Benchley said that a freelance is someone who gets paid by the word—per piece or perhaps. My problem isn't so much getting paid, but buckling down and getting the job done. BadLanguage.net has a list of 10 stratagems to do just that.
I love number three: "Greed and guilt. I remind myself how much money I’m getting paid for a particular assignment and how ashamed I will be if I miss the deadline. This actually works sometimes."