Put them in clear sleeves like these
. If you search around you'll find lots of sizes and mils available from a variety of companies.
If you don't want to mat them then use backing boards
and sleeve them, this will keep them neat and protected. I wouldn't waste money on high grade boards, you just need some that will keep the photos from bending. I don't know what size your working with, but you can get boards in more sizes than shown at the link above.