This is about as easy as it gets to get some good/free subscriptions, to publications such as the Wall Street Journal, Sports Illustrated, Golf Digest, the Economist, InStyle, and Golf Digest (and MANY more). Read on for more details ...
Update (10/23/14): If you're like me and you signed up for this deal almost 3 months ago, you may be about to lose the 8000 miles they gave you for free (unless you've flown Spirit or generated miles in some other way - like filling out surveys that earn miles - you NEED to have some activity with Spirit every 3 months or they cancel your miles). So, before they get cancelled, you may want to go to their website now and "spend" the miles on magazine or newspaper subscriptions!
I'm just going to give you two links (HERE & HERE), since these bloggers tell you everything I would anyway.
I will simply note that it will take you up to 10 days to get the free miles - that you use to "buy" the free subscriptions. Whatever you think of Spirit Airlines, I think this is a pretty legit deal. You could also just keep the Spirit miles in your Spirit account, but beware that Spirit miles unless you generate new miles on your account at least every 3 months.
(TRICK: There are some websites, like www.milesforthoughts.com, that will give you airline miles for filling out surveys or dining at restaurants. They don't give too many miles, especially for the shorter surveys, but they are a good way to keep accounts active on LOTS of different airlines - but most notably on Spirit, given their 3-month expiration rule. Most airlines have 12- or 24-month windows before their miles expire.)
UPDATED (7/26/14): Here are two more links (HERE & HERE) that also give you free magazines. Not as good of a sections, but almost NO effort. I have used the first link previously (and it worked) and I just found the second link (looks like it is from same website - but you might be able to use both PER PERSON).
Also, a quick follow-up to the Spirit Airlines miles (above). You may want to use them for saving and using towards flights on Spirit (instead of using for magazines). But there's a couple of big problems with Spirit's mileage program (if you want to use the mile for free reward flights). First, as I mention above, the miles expire every 3 months if you don't generate new miles (see my "trick" for easily generating a few new miles every 3 months - but you'd have to give yourself a calendar reminder to do so, and it might not be worth the hassle). Second, as you will see on Spirit's website, they have 2 different "schedules" for how many miles it takes to get a free reward flight - depending on if you have their Spirit MasterCard. If you get that (not saying you should), then you can literally get some flights for as low as 2500 miles for a one-way flight! But there are LOTS of restrictions (dates when you can get flights for that few miles). So, realistically, unless you are VERY flexible on when you are willing to fly (and you get the MasterCard), you're looking at using at least 10,000 miles for a one-way flight.
Thus ... a far more simple use for these free miles is getting the free magazine subscriptions! (If it were any other airline, getting 8000 free miles for free would be a great deal worth jumping on!)