I sometimes have to use a soft cotton rag with detailign spray for those models which dust has built up on.
GMP offers a detailing kit with polish, cleaning compounds and detailing spray. Their all Meguiars products and can be found at any auto parts store.
I have went through two of these kits and now I just buy the polishes and spray off the shelf at the auto parts store or any big box store that carries them.
Someone mentioned that they had soaked a model in Lemon Pledge for a week or so and then wiped the model off and it kept the model dust free. I don't remember the exact details of the process, so I wouldn't advise that before knowing the full facts. Do a search for 'pledge' and see what is yielded.
I think the "Pledge" was from me.
I have used it to polish up a few models that sit uncovered and it seems to do a great job of keeping the cars free from dust.
Just put it on a soft rag and wipe the car down with it. No need to soak it for a week :giggle
Most of my cars are in their boxes, but when I do need to clean them, I've found liquid spray wax to be the easiest for the job (perfect for a lazy bum like me). I've tried several products and like Mother's FX Spray Wax best.
You can get a 24 fluid oz for less than $6 dollars at any autoparts stores. :cheers