This article is from the Toys FAQ, by firstname.lastname@example.org (Scott Gordon) with numerous contributions by others.
From the rec.arts.sf.starwars.collecting FAQ:
"Your best bet to avoid yellowing and other damage is to store
your carded figures in comic bags with a comic backing board.
Place the board behind the card inside the bag and then seal the
bag with tape along the bag (taping the bag to itself). Comic
boxes make convenient storage units for carded figures stored in
these comic bags. Sealing carded figures in comic bags reduces
damage due to light, temperature, humidity, etc."