Pen Name
If you write under a pen name, this may be the best option for your website. If your real name is Bob Jones, but you write as Mickey Maverick, you could use www.mickeymaverick.com as your domain name. This will help you to establish and maintain your brand, and at the same time is easy to remember and write down.Real Name
This is the most common choice for writing portfolio domain names. Your real name helps establish you as a real person, not just some writing on a web page. Using your real name lets clients know you are serious and not afraid to stand behind what you do. If you have privacy or safety concerns, you may want to choose a different domain instead of your real name.
Other Options
If a pen name or real name either doesn't suit you or isn't available, some other domain choices can be effective. If you are known in online writing communities by a particular name, such as a forum user ID, you can opt to use this for your domain name. Be aware, however, that it makes it easier to connect words you say in a community to the professional writer you want to portray on your website. If you don't want to have that connection made, come up with something else.
You can also use a descriptive domain name, such as "thewebpagewriter" or "freelancewriter" (both of those are probably taken, but you get the idea). Above all, choose a name you will be happy with in the distant future, and one that will not limit your options. If you want to write web articles and also publish sales flyers, choose a name that will not contradict one or the other.