I've often found myself less inclined to write what I know, and more interested in writing the exact opposite. However, as I've learned to incorporate more of the familiar into my writing, I've come to appreciate the value of applying that same advice. Using places, careers, and interests that you know and love has three key advantages:
1. Believability. If it's a place you've been (or lived in), an interest you have, or a career path you're familiar with, your writing will show it and your readers will buy it.
2. Less research. Instead of starting from scratch with something you know nothing about, you can go into your writing with a foundation. You may still need to do research, but you may be more driven to delve into the subject.
3. Enthusiasm. We tend to put more heart into something we're passionate about. So if you use what you know and love, either as a starting place for your writing or to fill in details, your enthusiasm for it will seep through and affect readers.
If you often shy away from including what you know and love in your own writing, try the opposite. Consciously choose settings, careers, or interests that you're familiar with and/or are passionate about. It may take you in unexpected directions and to equally exciting places as the unknown.
Experiment with your work-in-progress or another piece that takes place in unfamiliar territory. Change the setting and place your characters where you currently live, a favorite vacation spot, or another place you know well. Write a new scene in this new setting and see what happens. Are the details more clear? Do you find your writing is less stilted and more free flowing? How does this new environment affect your characters? Aim for 2-3 pages.