"Write what you know" is up there with the writiest writey cliches along with "Show, don't tell." However, is writing what you know actually good advice? Jenna Glatzer thinks that there is more to it:

"Write what you know" is a very good starting point. But that's all it is. It's a place for you to go to get your feet wet, and a place to come back to when the tide gets too high. But it's not a place to stay for very long.

A better piece of advice, in my opinion, is "Write what you WANT to know." One of the great perks of being a freelance writer is that you get paid to learn about things. So... what do you want to learn about?

She has some tips on how to write what you WANT to know, without getting over your head, here.