make sure the brass is very clean, free of dirt, oil, polish and lacquer. You can use specialist undercoat spray sold in b and q (and probably other places) to make the paint stick better. If its an intricate piece, spray might be better than using a brush.
I did have to paint some brass this weekend, I sand blasted it and then dipped it in acid before painting, but I'm guessing most people can't easily lay their hands on a sand blaster and a tub of acid.
One of the best sprays I've used for silver effect is Plasti-kote Brilliant Metallic Spray Silver. It's almost chrome-like in its finish. B&Q and elsewhere.