I'm pretty sure that the only thing that works to "extend" the miles expiration date is accrual of miles - not the spending of miles (which I assume you're attempting with the magazine trick).

For some airlines, all it requires in some "activity" (any positive or negative change) in your account, so in that case, spending a few miles on things like magazine subscriptions works. For Frontier, I don't think that's going to work - must find a way to add miles to your account, not subtract them.