A coat of lacquer is the best way to prevent brass from tarnishing, and renewing the lacquer periodically can keep your brass looking its best. Because it's an alloy of zinc and copper, unfinished brass tarnishes quickly -- but the good news is that the tarnish prevents further damage and isn't difficult to clean. This means that lacquering a tarnished piece involves about the same amount of work as re-lacquering one protected by an existing finish.
Removing Existing Lacquer
Before you clean your brass in preparation for a new finish, it's important to remove any existing lacquer that has peeled and allowed the metal underneath to tarnish. Because lacquer emulsifies in lacquer thinner, which is available at any hardware store, either dipping your piece in that solvent or soaking a rag and using it to wipe the piece removes the old finish. Scrub hard-to-reach areas with a toothbrush. Acetone, or nail polish remover, is a suitable substitute for lacquer thinner. Be sure to wear a respirator and keep the space ventilated when using either solvent.
Cleaning and Polishing
Before you apply lacquer, you need to remove tarnish and vestiges of lacquer, and polish the brass so it will shine under the new finish. Use a commercial brass cleaner, but apply only a small amount to your cleaning cloth -- too much wears the brass and may remove brass plating altogether. Rub the piece vigorously to generate heat -- this helps loosen the chemical bonds binding the oxidized material to the surface. After cleaning the brass, buff it to a gleaming finish using a second clean, soft rag.
Before coating your brass with lacquer, you may want to darken or burnish the finish, or even give it some color to make it look more antique. There are several chemicals you can use to do this. For example, immersing the brass in a water solution of copper sulphate will give it a dull green hue. Dipping it in hydrochloric acid and ferric oxide will dull it, and putting it in a water solution of nitrate of iron and hyposulphite of soda will give it reddish brown tones. Many more colorizing recipes -- some of which require care because they involve strong acids -- are available.
Coating with Fresh Lacquer
Solvent-based nitrocellulose lacquer is the best type to use for coating brass. If your piece is small enough, the quickest way to coat it is to dip it in lacquer and hang it to dry. The alternative is to spray the lacquer; for this, air spray equipment and an aerosol can are equally effective. If the brass is part of something larger -- such as a brass doorknob -- protect the surface you aren't lacquering with masking paper and masking tape. Spray one coat of lacquer, sand it lightly with 400-grit sandpaper, and then spray a second coat.