If you find some publications you'd like to write for, read a few issues very thoroughly, look at what they've covered in the last six months or so, and see if you can think of something you know about, which you could write about, which would be new for them. And then just write a very brief email to them, explaining why you think it would work for them, and why you're the person to write it.

It might not be the best approach, but it's worked for me more times than I can remember.