AP Stylebook? I don't need no AP Stylebook.

Miko and thinkingwoman give some great advice. I think there are times when using first names is appropriate - like for very long, feature stories - but I don't know what the tone is of your article, if it is more featurey or more newsy. And I am surprised your magazine does not have some kind of style guide. If you don't want to look like a "hick rag," then you guys should either make an in-house style guide or use something like the AP style guide. Consistency is key and you will look more amateurish if you don't have any.

Using her first name implies an intimacy that does not, in fact, exist. (Unless you're closer to the subject than you've already said.) You want her and your magazine to seem approachable? Do good reporting that goes beyond the superficial. Ask her questions that get her to share with you, and through you, the readers.

... is a good one. (That's how that last sentence should read. Sheesh!)

As a woman, if you did an article on me, especially if I had a big important job or life worthy of a profile, I'd want you to use my last name. It is more formal but it's what all the guys get. Maybe it depends on the piece but I'd be inclined to play it safe and respectfully. And I think the advice to refer to husbands and brothers and friends by their first name.

I think you should use the last name. First name is odd. The biggest problem with AP style (and the best thing about the Times) comes up when you have siblings, or married couples, etc, because it's really a drag to say Jane Smith and John Smith every time instead Mr./Mrs./Ms., etc. Since you're not going with AP style, necessarily, I really like thinkingwoman's suggestion about first name for relatives once you've introduced them. It sounds nice. If you really think a first-name basis is alright, then you should have a rational reason for doing it. (For instance, if you know she LIKES being called by her first name, this is how I would start the article. "Mary Smith Jones is the president of X university, but she's on a first-name basis with almost everyone who knows her. Mary ...." etc.) If this is not true, then you should stick to the last name. The only time I find the first name use compelling is when dealing with unspeakable tragedy, or when someone is handicapped, or when someone is a child. If you use the first name for an otherwise fully functioning adult, you are telling the reader that there is something to pity or pardon about them.

Our style here is honorifics except for criminals -- which was common in ye olden days.

Thinkingwoman's got it. There are much better ways to lighten tone than referring to a subject by first name throughout a piece.

Miss/Mr./Mrs is considered pretty archaic in modern American periodicals. Honorifics have generally been dropped except for a few holdouts, notably the New York Times (which uses its own unique stylebook, not AP). One of the main reasons for that is that honorific usage isn't parallel unless you go with the nonsexist "Ms." universally, which always raises objections, or you go with "Miss/Mrs." which is sexist. In general, last names are standard in professional writing across the vast majority of publications. A few minutes at any magazine counter will bear that out. The only exceptions I can think of seeing in recent years are in freebie, 'shopper' ad-carrier weeklies, and unfortunately the use of first names seems to make them sound even more rinky-dink and parochial.

I won't keep arguing this point, but "respectful" is not the only tone implied by using last names, especially in features, which is what this sounds like. I've had readers complain they sound like criminals, or at best very cold and distant impersonal figures, when referred to with last names only due to my publication's style. If being respectful in a formal way is your key priority, you might as well go all the way and refer to "Miss/Mrs/Ms" Jones.