Every now and then I stumble upon a rare set on eBay that ... I must have. Once it arrives, it must get 'sanitized'. What I do is, disassemble totally, immerse in hot water, and pour a generous amount of dishwasher soap on the parts, then I mix/stir the whole thing few times and let it 'bathe' overnight. The dishwasher soap has a tendency to 'eat-up' the dirt so, in general, no brushing is needed. I've 'read' some people throw the whole thing in a clothes washer ... but with small technic pieces, one needs a fine mesh bag of the sorts. This also applies to assembled sets that have been sitting on a shelf for some time and accumulated few microns of dust as well.
What 'technique' do you use ... and have found to work best?