This mistake can take two different forms. The first is simply submitting written work in the form of articles and stories to various websites for no pay at all. There are many websites out there that will allow you to post your own work and make a little money. Even if that money is small, it's better than nothing.
The other trap aspiring writers fall into is the promise of pay at a later date. There are dozens of companies out there who will tell you that they can't afford to pay writers right now, but they'll get to it in the future. This is never true. NEVER. Even well-meaning companies don't follow through. Why pay for something you can get for free? If you volunteer your services, even once, don't expect that company to ever pay you. It simply doesn't make good business sense.
If you really want to be a professional writer, you have to treat yourself as a professional. Write for a fee, not for free. True, there may be instances where you want to donate your time, such as editing the newletter for a charitable organization, but these should be few and far between.
