The most excellent John Walker has written a wee bit of advice for aspiring games journalists, and I reckon much of it is relevant to all kinds of freelancing.
This job is a not a privilege. It’s something you got by being good at what you do – you earned it. Anyone who tells you it’s a privilege is trying to get something from you they shouldn’t have. That’s the language of those who want you to do just a little bit more work than they’re paying you for, or put up with conditions that don’t feel appropriate.
…Make a fuss. Good grief, the number of times I’ve not been paid for work, or screwed over in some way, is awful. It’s generally down to incompetence rather than malice, but it’s unacceptable.