Unlike Google Adwords or any online ad buys in general, how do people track effectiveness of a magazine ad? Are there some strategies you can use to figure out which print ad is working the best (A/B split testing print ads).
Most of what I've seen is using custom links or phone numbers for each major ad variation / publication. http://oursite.com/economist, http://oursite.com/newyorker, etc.
If you're targeting an audience that will use QR codes, that gives you even more options as you can put a link with multiple parameters into the QR code: