Too much work to read through all of these responses, so I apologize if I'm rehashing this: I use stick-on diopters on my non-presciription sunglasses. Several locations on the Web sell them, like this one:

http://www.stickonlenses.net/servlet/StoreFront

I use just one (the other eye accommodates); it's possible to cut down the size of each stick-on lens, too, which makes them work with glasses that are too curved to hold the entire diopter.