If you have any tomatoes that are almost past it, you can use the juice to soak smaller brass items. Place the juice into a bowl, them soak the brass for around five minutes (for very dirty brass you may need longer). When done rinse with warm water and again dry very well and buff.
The less you touch your brass items the less they will tarnish from the natural oils in your hands. To help prevent your clean, shiny brass from tarnishing again, rub over a thin coat of linseed oil.