I have been working as a freelance contributor for a little over a year now. In this time, I've picked up on a few unprofessional things freelancers tend to do. Based on said learning, here are a few ways to maintain professionalism even if you're only a freelancer.
Always maintain deadlines
This is perhaps the most common issue I've noticed among freelancers - we don't stick to deadlines. This reflects poorly on our work ethics. All work should be taken seriously and done on time, even if one is not a full time employee.
Choose quality over quantity
The quality of my work is often subpar if I procrastinate until the last minute and prepare something in a hurry just to make it within a deadline. Quality always matters, and poor quality work only adds incompetence and sloppiness to a freelancer's reputation.
Keep your commitments
As freelancers, many of us tend to be irresponsible with commitments. Not showing up to monthly meetings is a common example of this. Promising to deliver something and neither delivering that output nor giving a heads up to your employer is another unprofessional thing to do, and it basically tells your employer that s/he cannot rely on you in times of need.
Be available for feedback
This is a less common issue, but problematic nonetheless. Sometimes I'll write an article that needs a few edits before it can go to print. If I choose to not respond to my employer's calls and texts in such a situation, the work either gets delayed or has to be done by someone else. It's true that we're not personally inconvenienced by such events, but it's also true that reputation and trust are important in getting freelance work. Hence, try to be available when there's a chance of receiving feedback.