I usually take the bag w/ cereal out(carefully). I try to open the box as carefully as possible(the cardboard tends to tear when you try to open it. They sell plastic protector cases(usually pretty high price) that protect the box and make a fantastic display.
Some collectors open both ends and flatten the box out, this works as well and takes up a lot less storage space.
Testimoial-I work in the food business and about 2.5yrs. ago I found a Corn Pops box w/making of SW offer on it in a kitchen at one of the restaurants in town. It was sitting there for quite a few years. I asked if I could have it, and when I opened it the cereal was all stuck together in one big lump(it actually was kinda melted together) the box was in decent shape and after a quick mild wipe it actually looks great! You have to realize this box was in a greasy heavily used kitchen for yrs., so you shouldn't have to worry if you simply put the box on a shelf in a room and dust every now and then.