The bottom line, as I quickly learned in my first few days on the job at a museum, is that once a model (or any other artifact) gets dust on it, it's almost impossible to bring it back to exactly what it looked like before the dust settled. The only genuine solution is to stop the dust from settling in the first place: keep the model in a case of some kind.
I like to build cases for my larger models. (My choice materials are cherry and plexiglas.) for the smaller ones, my wife and I bought, about 20 years ago, a "curio cabinet." It's wood and glass with a hinged front, glass shelves, and a built-in light. Twenty years ago it cost about $200 in a furniture store. (The same furniture store where I experienced the biggest humiliation of my life: I got seasick lying on a waterbed.)
The curio cabinet does a good job under normal circumstances. The day we had a carpenter in sanding the spackle on the ceiling, and forgot to cover the curio cabinet, was another matter. A couple of those poor little 1/700 warships are never going to be the same.
None of this does any good for a model that's already dusty. For that, about the best you can do is blast it gently with an airbrush - after doing the best you can with a soft brush. You can also try putting a little clean water in the airbrush.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.