When you are not wearing your silver jewellery, keep it in its packaging to limit exposure to the air, which causes oxidasion. It's a good idea to keep an anti-tarnish strip inside the box too.  It's an inexpensive paper strip, impregnated with anti-tarnish solution (Town Talk is a good example).  When the silver gets a little dull, it's the time to clean your piece - the longer you leave it, the yellower and darker it will become.  Wash with warm soapy water and dry gently with a soft cloth or paper.  For fiddly pieces you can use a soft toothbrush.  You can also use a polishing cloth (Goddards or Town Talk are good).  Rub the piece gently and it will restore the shine.  If the tarnish is too heavy, you may need to use a silver dip, but it's quite a harsh chemical, so use sparingly and dip quickly in and out; rinse immediately, dry and then rub gently with the polishing cloth.