In short, the answer is: You need to write. And then write some more and then some more on top of that. If you are lucky enough to have a job that requires a lot of writing, you can build a significant portion of your portfolio with your work. But what about those who are trying to build a portfolio without much prior experience?
Again, the answer is to write. If you have ever had anything published – and let’s not be shy here – anything that has been published on the web is acceptable – highlight that. Newspaper clippings are great. Heck, you can even use well done school projects and contest entries if you have nothing else.
Be creative too. If you don’t have any clients, that’s no reason not to produce something. For example, you could pick a company or cause or whatever and design a “campaign” around it. Include a cover story, a brochure, perhaps a front page of a website…this could be some of your “sample” work.
As you fill out your portfolio, look for ways to diversify the material because a wide range of writing styles enhances your overall chances of success. Some people have categorized their portfolio by specific genre – others have included a “Table of Contents” where each “chapter” is a different writing style.
The point is, a portfolio showcases your work, backs up your resume and proves that you are a committed writer. That’s why the only real way you are going to have a top notch portfolio is to write and write often about a wide range of topics.