I use Mylar 4mil sleeves. Stored flat. Some I put in art presentation folders. Some I keep in safe deposit box. I have a few in frames as well, but most of those I display are prints of the originals.
These are the ones I have used. They are sold for comics, but work very well for photos. They come in different sizes as well.
https://www.ebay.com/itm/500-E-Gerbe...IAAOSw~-Zce~U7