Clarify that comment is not for free
The Freelance has heard reports of freelance journalists who have contributed to the Guardian Comment is Free blog being asked (retrospectively) if they could "do it pro bono" - which is Latin, we are told, for "without payment".
The Freelance's own assistant editor pitched a story based on their own experiences to the Guardian, and was rung up by the commissioning editor asking some questions about the feature proposal. Some minutes into the conversation came the uneasy sensation this was becoming an interview for a story about us, to be written by the Guardian staff journalist. Emails to said commissioning editor - and some of their colleagues - clarifying the freelance's position resulted in a paid-for commission.
The best way to avoid any unpleasant misunderstandings here is to make it clear from the outset (in your written pitch) that you are offering the article on the basis that you're a professional journalist who expects to be paid.
Citizen commenters too...
The Freelance editor is from time to time contacted by non-journalists who have written pieces about subjects they love - perhaps the most open to exploitation. They're not going to get a fair hourly rate for the work they put in to the films of Akira Kurosawa, or whatever... but they (those who contact us, anyway) are shocked to find that the Guardian expects them to assign their copyright for no fee.
In every case we've been involved in, the contributor sending the commissioner a link to the Guardian Freelance Charter, highlighting the bit that says how they aim to treat contributors, has resulted in payment.