From Good Housekeeping....

"squirt a dollop of ketchup on a rag, and rub over surface. Wipe off residue with a damp cloth and buff dry. For a homemade cleaner, dissolve 1 teaspoon salt in 1/2 cup vinegar and add enough flour to make a paste. Rub onto the brass and leave for 10 minutes; rinse and buff dry."