Cut a lemon, peel it, take a chunk of peel and sprinkle salt on the inside of it. Rub the brass with it. When it's clean enough, turn it over and rub the exterior of the peel on the brass, the oil protects it.
If the brass is really grubby, leave a bit of the pulp in the peel, if it's not, you probably don't need it. Too much acid eats the details on your brass down if you do it too often.