Some are in original boxes, some are in a glass display and see very little outside air exchanged, and many are in those plastic display cases you see for sale at TRU. I was grabbing all I could when I saw them on sale a few years back for $5 each.
I have tried a few of these methods and have found the following:
Used California Duster and eventually the dust will cake on edges of car so you will have to physically remove the dust at one point or another.
Used Nu-Finish made the cars extra clean and shiny but took hours to do just a few cars. Remember have to clean insides & tires also.
The glass casing or curio cabinets can't keep dust out. I notice that my cars in the closed curio cabinets still collect dust on them just not as bad as if they were just left out.
You can leave the cars in the boxes but what fun is that if you can't view the diecast in full?
Maybe good if you are a collector who is waiting to cash in $$$$ at a later date.
I do have cars still in their boxes just because I don't have room to take cars out and put the boxes away as that is more stuff that needs to be stored and will take up space that I don't have unless I buy a bigger house.
Up to this point I have found using a microfiber towel to be the easiest, fastest and most effective way of dust removal and cleaning.
I,ve got one og those black magic brushes that you use for your cars interior.Got it a wal-mart.Also use a cloth that you can get at computer stores. I think its a micro fiber cloth.The trick is to dust them at least every other day.
I go to Pepboys. There is a yellow flannel feeling detailing towel they sell (turkish cotton?! don't remember). Anyway, they are very soft and great for light dust. They are easier to handle than even a small duster. I use the same towels on the 1:1...for buffing off wax.
I use a micro fiber towel and DCW,die cast wax, I not sure how well it works but it resists fingure prints and is suppose to repel dust, and is easy to use
I use a can of compressed air, works great. I also have a small "feather duster, but made of plastic thread, very delicate on the cars.
I found I did more damage trying to clean them with a cloth, many have antenna's etc (C5Rs exoto's GS).
Mine are all out on display, they get pretty dusty, but I can't really appreciate them in the box, and the plastic displays take up more room than I have.
I really think the compressed air is a good choice, I haven't broke anything yet. But if the dust is thick you may need the feather duster.