Pretty much by definition, a professional gets paid to do whatever she is a professional at. Those of us who do something without getting paid may be amateurs or hobbyists or enthusiasts, but we aren't professional until we receive money in exchange for our work. The dictionary isn't quite that tidy, though.
Sure, I may be an amateur writer in the sense that no one has paid me a shiny nickel for my work (yet!), but that doesn't mean I can't approach my craft with professionalism. Professionalism (according to our word-defining friends) refers to "the skill, good judgment, and polite behavior that is expected from a person who is trained to do a job well."
As to skill, I won't claim to possess any particular skill, but hopefully I can demonstrate an occasional insight or clever turn of a phrase. I can show up regularly to demonstrate whatever skills I possess; showing up may be a skill in itself.
Good judgment may be rather like the inverse of obscenity: you know it when you don't see it. Be that as it may, I can at least refrain from being an idiot, whether I am getting paid or not.
Polite behavior may be the key to the entire endeavor. That doesn't just mean don't be a jerk, although it at least means that much. Polite behavior also means meeting your commitments, as well as not making commitments you can't meet. Polite behavior even means being affirmatively helpful when you can. A professional helps a colleague with a critique or a compliment or encouragement--without an expectation of the favor being returned.
I have a suspicion that I cannot yet confirm: that those who write with professionalism have the best shot at becoming professional writers. My suspicion may not be correct, but I choose to conduct myself as a professional regardless, whether or not I am getting paid.