Your business manners and communication skills must be top notch, it's always a good practice to make your clients feel secure in your skills and business practices.
I believe that it's poor practice, not to mention bad for our industry to offer freelance or contracted work at pro-bono. It devalues our work that some businesses or orgs believe they can get stellar designs for free (that's what internships are for right?). While it's difficult when designers are starting out, and need to make money, they NEED to make a living to support themselves as well as place confidence in their work by making their products worth something.
It's good to be able to cater to your clients' needs without sacrificing the integrity of your work. Part of the business means building a business portfolio. You don't want poor work you've done in the past to bite you in the ***. Work with your clients to cater to their needs, but also be willing to push your training and experience by constructively helping your clients to understand what makes effective design.
Lastly, study up; as a freelancer you'll likely be designing AND producing your work. Know your software and scripting (Javacript, CSS, DHTML, XML). Keep up with industry trends. If possible, and if you can afford it, join
AIGA or a local design association and get your work out there. One of the best things about the business is marketing your clients because you're also marketing yourself.