A hot soak in citric acid will remove real grimmy stuff.
For general use, walnut shells with a small splash of mineral spirits and a cap full of "Nu Finish" car polish.
You can find this wherever car wax is sold. Add the spirits ans polish to your walnut and let it run for about 30 min. until it is mixed together well. Add your brass and tumble for 4-5 hours. Your brass will look like new.