A portfolio in writing is similar to what a model uses to showcase their talents. Make a habit of saving copies of all of your work. A good portfolio will change over time with your experience and the number of successful jobs you complete. The ideal portfolio will have works from a variety of areas. It is also a good idea to index your work so when you need to adjust your portfolio to bid on a specific type of job you will have no problem finding what you need.
The first step to designing a bid winning portfolio is to decide what you will use it for. If you are looking for work in a magazine then you should have a portfolio of those types of articles. If you are building an portfolio for article marketing then you should have perfectly SEO optimized articles in your portfolio. It is always a good idea to adjust the portfolio to highlight the work that is most like what the employer is looking for.
When creating your portfolio it should be as professional as possible. Potential clients want to see at a glance what is inside. As they dig deeper they are looking to see if you write with continuity and that the style is coherent. Only use your finest quality work in your portfolio. Remember you are showcasing your abilities. You are essentially selling yourself and your talents.
The portfolio is your calling card, it is your initial interview, the first contact, make sure you give a good first impression, you will not get a chance to make a second one.