Yes really!  Good old tomato sauce is brilliant at cleaning brass as the acid in the tomatoes removes dirt and tarnish. All you need to do is get a soft cloth, pop some tomato sauce onto the cloth, then rub it well into the brass. Run under warm water to remove any residue, then dry thoroughly and buff.
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.