My Gran always had a tin of Brasso in her cupboard and swore by it for keeping her brass letterbox gleaming. I suspect it may have been the same tin throughout my childhood as a little bit of Brasso goes a long way!
I find that my brass letterbox tarnishes very quickly, even though it was originally sporting a lacquered finish that was supposed to reduce tarnishing. I had to rub the lacquer off and then polish the brass underneath, which was a particularly tricky job, but the brass has been much easier to keep clean subsequently. You apply the polish with a cloth, which blackens almost instantly, so you need gloves (washing up ones are fine) and something to protect your table. You do still need to use a fair bit of elbow grease, but I have found Brasso works much more efficiently on marks with a bit less effort than other cleaners I have tried, such as Cape Cod metal polishing cloths. The polishing cloths are sufficient for a minor spruce-up but they don't get used here for much more than that.
I suspect there will always be a tin of Brasso in my cupboard, but Grandparents do pass on valuable information so that's no real surprise.