First of all determine whether the object is really made up of brass. You can do this by holding a magnet to the item you want to clean. If it attracts the magnet then either the object is made of a metal other than brass, or is just brass plated. If it is brass plated then do not use any abrasive cleaning technique because the plating is usually quite thin and you could wear through it.
If the item is really made of brass and it needs to be cleaned then soak it into the aqueous mixture of common salt and white vinegar.
Pour in half cup of white vinegar in a container or bucket. Add one tablespoon of table salt to it and add enough hot water to cover the brass piece. Soak the brass item in it for 15 to 20 minutes.
Remove the brass piece from the container and gently rub it using soap-pad. Keep rubbing the item until you see the shine come through.
Rinse it under excess of warm tap water to remove soap residue.
Dry the item immediately with a soft cloth to prevent water spotting.
If the brass piece you are treating is purely used for decorative purposes then you can also use clear enamel spray to preserve its shine.