What kind of writing samples should a freelance writer have in their portfolio? What do employers usually look for?
The best, and maybe quickest way, is to learn to write really good, deep, thorough blog posts and then find places where you can submit as a guest blog poster. I would recommend learning everything you can from the trio of Carol Tice, Kristi Hines and Jon Morrow. Those are the places I go. Add in to that Copyblogger and you will be on your way.
Sometimes you can guest post before you even have your own blog. However, if you can, set up your own blog first so that you have a URL for your byline.
I think what employers, who are worth their salt, look for is a writer who have some good, solid articles on some good sites with quality traffic. Is there a standard bearer blog in your industry? That would be a great place to start. If you are able to get a guest post there, then it will really help.