If your sterling or gold collar does tarnish we recommend using a good silver polish or dip.  These items can be purchased at just about any hardware store or supermarket.  Our favorite is a product called 'Tarnex' (see video below)  It is literally a dip (or wipe) for 5 seconds and the tarnish is magically gone!  Just remember that Tarnex works only on 'tarnish' and you should clean your collar or lock with soap and water first to clean off any dirt or oils first before using the Tarnex.  Also remember to rinse the collar or lock after using the Tarnex.  Then just wipe dry with a paper towel or soft cloth/towel to shine.
There are many good silver polishes out there as well.  Many of these have mild abrasives that can be used if your collar is lightly scratched.  Always follow the manufacturer's directions.
There are also many 'jeweler's cloths' out there that are wonderful for a quick and easy shine.  Many are formulated to remove tarnish and polish as you wipe.  Again, always read the manufacturer's description and instructions before using on your special collar.
TARNEX VIDEO: