I just use a 1" to 1-1/2" paint brush--technically a "trim brush," I think--with soft, fluffy bristles. Natural or "simulated natural" bristles are best (i.e., not the cheap, throw-away brushes with stiff nylon bristles in the hardware store bargain bins). You have to be careful using it around pitot tubes, probes, antennas, gun barrels, etc., but you can get a model clean pretty quickly with one.
Of course, a display case is the best way to keep your models from getting dusty. If you've got the space for one (and the means for lugging it home), you can sometimes get one fairly inexpensively when a store goes out of business and sells off its fixtures.
If you must display your models in the open, it's a good idea to dust them regularly. Dust attracts moisture from the air, and over time it can turn into a sticky, gritty film on the models that's a lot harder to remove.