The tarnish on guitar hardware occurs due to a reaction between the plating and moisture; either in the air or from perspiration. Keep in mind that all hardware will tarnish eventually and is not a sign that your hardware is “cheap.” In some areas of the world there is low moisture content in the air so the tarnishing process will be slower, but it will eventually happen. There are a lot of guitarists out there who consider a little tarnish on their guitar as a sort of badge of honor. That said, you can slow down the tarnishing process by wiping down the instrument every time you play it. This one step will add life to the hardware and slow down the tarnishing process. Also, make sure you store the instrument in its case to prevent it from exposure to high moisture content level in the air.
Moisture can be a problem in an enclosed space like a case; however, one should be careful with the assumption that dry conditions are good for an instrument. If the air gets too dry the wood and finish can be at risk of developing cracks. A typical home in the winter time – with furnace running and no humidifier – is too dry an environment for a guitar. If exposed to this type of climate for too long problems will develop (though your hardware will tarnish less). Generally it’s a good idea to keep guitars in environments that are comfortable for humans – not to moist and not too dry. Wipe off the hardware regularly and hope for the best.