Just for personal use, and not any sort of support for Amazon's business practices:
When you get a free (or cheaper than you're currently paying for shipping) Prime offer, you can accept it and then the second the order has gone through go into your account and cancel it and still get the remaining number of promotional days, as well as the benefits for the current order. It's a slightly annoying step, but I figure if they're going to be dumb enough to keep offering me free promotions I might as well take advantage of it.
When you get a free (or cheaper than you're currently paying for shipping) Prime offer, you can accept it and then the second the order has gone through go into your account and cancel it and still get the remaining number of promotional days, as well as the benefits for the current order. It's a slightly annoying step, but I figure if they're going to be dumb enough to keep offering me free promotions I might as well take advantage of it.