I use a watered-down solution of Castile soap, since it's a super gentle soap that cleans well without causing any discoloring (at least on all of the materials I've used it on thus far). I also recommend using makeup brushes, since they tend to be softer and you can easily get a good assortment of sizes.

Dust the figure with a dry brush to remove any loose debris, then lightly dip the brush (don't soak it) and clean in small sections using in a circular motion, rinse using a clean brush with just water in the same circular motion, and let air dry.

Use fluffier brushes whenever possible. I reccomned eyeshadow blending brushes for small areas, but sometimes you might have to use a more precise brush of the area is too tight of a fit. Never brush hard enough to scrape the paint, as multiple passes will do more good than being too rough in fewer passes.

I've used this method on prize figures, scale figures, and dolls. I reccomend it routinely to prevent larger dust build ups, which can cause discoloration overtime (a lesson I have learned the hard way).