s/o - what do you do with old photos?

[sweetptater]

The Displaying Family Photos post below got me thinking - what do you do with the previous photos that were in the frame when you replace them? Currently, I put the new photo over the old photo and leave all in the frame. This has always worked in the past since I had very few photos on the walls and rarely changed them out. Now that I have two kids, I have pictures hanging everywhere and between professional shots and their daycare pictures, I'm getting new pictures at least 2x per year. My stacking method isn't going to last much longer.