To borrow from Nike: "Just do it."
I'm not a publisher, but I've gotten a few things published. Here's my take.
Look at the guidelines for the magazines you're interested in. Most (if not all) have some kind of specifications as to what they're looking for in terms of article submissions. When something strikes your fancy ("hmm, looking for articles on new spells and how they reflect their caster's personality..." sounds good!), study that mag's contents from a few different issues. This gives you a feel for what they like. Perhaps not so much in terms of 'how' to write it, but you know if they like over-the-top approaches or a more conservative style.
Should the articles seem to have a certain format (and the mag's guidlines might be quite specific on the matter), then follow it as best your subject allows. And then write, revise, write, revise, etc. until you are happy with it. Does the article read well? Try reading it aloud. Does one section flow logically into the next? Good. Have a general overview for your intro and wrap up the article by summarizing the conclusions you've drawn and expand (if possible) on tangents that the reader can follow up in his own game.
I'm sure that others will chime in here, but that's about all I can think of at the moment. And make sure to FOLLOW INSTRUCTIONS. If the magazine/company has guidelines, follow them to the letter. If you do not, chances are good you're article will be tossed away in favor of one that is formatted properly.
