Magazine editors want to see examples of your work when you approach them with a story idea, so start writing for local publications, student newspapers, or Web sites to get some experience before contacting the big names.
Any time you manage to get your name in print, whether it's for a company newsletter or a specialty magazine, make lots of copies and keep a file of your work. You'll want to organize the file according to the type of writing it contains. For example, business or marketing content is quite a different thing from a human-interest story. When it comes time to send clips to an editor, you must be able to match your clips to the article ideas you are proposing.