There is an excellent article on nickel (and copper) plating here. When I used the method on brass fittings, I found that the nickel plate on the copper wire that I used to hang the fittings was better than the plating on the fittings. Initially I decided to copper plate the brass first as a substrate for the nickel, and this improved the coating - but the method below is easier. Produce the finish you would like on the brass component (rough, smooth, polished). The nickel plate will 'copy' this finish. Clean the brass component and THEN soak in a citric acid solution until it goes a slightly reddish colour. Rinse and dry, but don't be tempted to re-polish. Then nickel plate as normal and you should get a better plating on this copper-rich etched surface than on an untreated brass surface.