First of all, Northwest miles don't expire (I still have the 2544 miles I accrued in 1992 with no activity since except setting up access on their website a couple months ago!), so no need to spend them on magazines if you're already 4/5 of the way to a free ticket.

One good way to add to Northwest miles gradually is to look into the World Perks Mall at their website. Basically, if you go to a number of well-known websites (inlcuding barnesandnoble.com, timelife.com, jcpenneys.com, etc) through links at the World Perks mall when you're ready to buy something from one of those sites, then you earn Northwest miles on those purchases (in addition to any miles you earn anywhere else by whatever credit card you use), and it doesn't cost you any more than if you went directly to those websites. (I'm not saying you need to do all your shopping there, just remember if/when you DO get around to thinking of buying something from one of those websites to link to it from World Perks Mall.)

You might want to look around the websites for the airlines where you do face expiration issues and see if there are any similar shopping links where you earn miles at stores you wouldn't mind shopping at anyway even if you didn't earn miles.