DMGF*ck you. Pay me.
WARNING! There's a bit of profanity in there. But I think it's in the fine tradition of making a point, building a bit of self-confidence, and making the message memorable.
Also, I find it really, really interesting how often freelance professionals simultaneously trumpet how valuable their services are (programming, creative services, catering), while ignoring the value in others' services (lawyers, accountants, investment advisors, personal trainers).