If you don't already, subscribe to writers periodicals such as Writer's Digest. There are many to choose from just put your google skills to work or visit your local library. With the major presence the Internet has in our lives, the library seems to fall by the wayside. It's a great place to do research both on line and by browsing rows and rows of the written word.
Once you peruse the magazines, digests, and trade magazines (the ones specific to your areas of expertise), see where to contact them and submit your work. In my experience, the trade magazines are easier to break into when you have credentials. I started writing for a telecommunications publication when I was selling and scripting in-house voice mail systems for businesses. I wrote a monthly column, sort of the Dear Abby of phone systems!
Once you find out where to submit, get to work. Submit your writing and don't take no for an answer. I've found that I don't get told no, I just don't hear anything at all. I still submit work to the national magazines and though I've not landed that prime real estate yet, I will keep trying.