All precious metals will scratch and tarnish with general wear and tear - the general tips will help you prolong the life of your jewellery but there will come a time when it will need a bit more TLC.
A professional jeweller will be able to get rid of nicks and scratches for you at a price - however avoid doing this too often as it will wear down the metal.
When your sterling silver jewellery becomes tarnished you can wipe it with a silver polishing cloth - be careful if there are stones and beads on the piece.
An ultrasonic cleaner with a small bit of washing up detergent is also excellent at cleaning sterling silver jewellery and they are widely available - however if the piece contains gemstones and beads seek advice before using it as not all gemstones and beads can be cleaned in an ultrasonic cleaner.
Very dirty pieces should always be professionally cleaned by an experienced jeweller.